a lot more of the terms used for weapons are more... ambiguous.

Yes, this is very much the case. In most places and most times, the common varieties of sword were usually called "sword". Many of these terms have been imported from the local languages of those times and places and are used in English today to mean a particular type of sword (e.g., gladius, spatha, xiphos, parang, shamshir, tulwar, and more), but they originally just mean "sword", generically.

Where swords of different sizes were used, terms like "short sword", "sword", and "big sword" were sometimes used (e.g., Japanese tanto or shoto, katana, and tachi or daito, respectively), or terms describing how the sword is worn (Chinese peidao and Japanese wakizashi, "waist sword"). Swords used in one hand or two can be distinguished by "sword" and "two-handed sword". Single-edged and double-edged swords are often distinguished from each other (Chinese jian and dao, English sword and backsword, Greek xiphos and kopis or makhaira).

The finer classification of types is something mostly done by modern collectors (and militaries after the advent of uniform patterns of arms), rather than by those who used the swords originally. Perhaps the ultimate n detail is the classification of the Indonesian-Malay keris by collectors, with what is to outsiders a single type of blade (a keris) being classified by details of the patterns, blade shapes, etc. (Harsrinuksmo, 2004). However, there are some detailed classfications used in the past. For example, al-Kindi classifies swords by material (iron, pattern-welded iron and steel, crucible steel), and/or place of origin, and type of pattern on the surface of the blade (Hoyland & Gilmour, 2006). Japanese blades are classified by cross-section of the blade (e.g., shinogi-zukuri, hira-zukuri, etc.), shape of tip, curvature of the blade (whether uniform or concentrated in some part of the blade), pattern of hamon (hardened edge pattern), and style/school of swordsmith.

What was the unique purposes behind certain distinctions? What, if anything, makes this weapon or specific weapon part/type better or worse then similar weapons?

Modern classifications are to serve modern classifiers and collectors, and can often be ignored as far as function goes. Some old classifications are related to quality - e.g., al-Kindi describes the quality of his various types of swords in terms of sharpness, toughness, etc. Other old classifications have very little to do with quality, and are more for the same purposes as modern collector's classification.

The very basic classifications as "small, medium, large" and "one-handed and two-handed" do affect function. Things like one-handed vs two-handed use make a difference (e.g., shield or no shield), and length and weight matter (since these affect reach and speed). Not all of this difference is captured by the usual classifications. For example, the typical Viking-style sword weighs 1.2kg, but there are surviving examples as light as 700g and as brutally heavy as 1.8-1.9kg - this variation will greatly affect the handling of the weapons.

"Better/worse" is a trickier thing. If one compares the weapons on the basis of having them ready at hand, and fighting one-on-one on an open level area, things such as reach become very important (although armour will mitigate the effect of reach). Thus, it is no surprise to find that the dominant hand-to-hand weapons on battlefields are typically spears or other long polearms. However, despite this, one finds that swords have been common sidearms. The weapon you have with you because it is convenient to wear at your side is better than the weapon you left at home because it is difficult to take everywhere with you. (Similarly, pistols/revolvers can be "better" than rifles/muskets due to ease of carry.) Similarly, a one-on-one duel differs from the battlefield - on the battlefield, large shields are useful for stopping arrows etc., while duels without arrows often favour small shields (e.g., bucklers) or parrying weapons such as left-hand daggers.

Many, many words have been expended on debates such as "katana vs longsword", which of two particular sword types is "better". If one keeps in mind basic principles of length, weight, handedness, and perhaps curvature, much of such debate can be ignored:

• Length: reach matters a lot, especially if no armour is worn. If the two fighters start apart, the one with the greater reach (usually the fighter with the longer weapon) has the initiative. However, longer is not always better. Too long can make the weapon difficult to use in close, and longer typically means heavier (and therefore slower). That great rater and ranker of weapons against each other, George Silver (1599) describes the ideal length as "perfect length". His perfect length varies for different weapon types, but for those he ranks as the most effective (which are two-handed polearms), it is about 8 feet.

• Weight: speed matters, and weight affects speed. Too slow is bad, and therefore lighter is often better. However, it takes more skill to cut effectively with lighter weapons (e.g., a 600g sabre instead of a 1kg sabre), and a heavier weapon can be more effective in the stress of combat (when a lot of fine technique flies out the door). A too-light weapon can also be fragile.

• One-handed vs two-handed. One-handed weapons allow shields or off-hand parrying weapons to be used. The importance of shields depends on whether missiles are in use, and whether and how much armour is worn. Two-handed weapons provide more leverage (but not necessarily more speed). One-handed weapons can provide more reach for a given length (but two-handed spears can provide more reach for a given length, since they can be gripped closer to the butt).

• Curvature: a curved blade can make slicing draw cuts easier (they're certainly possible with straight blades, but there is the issue noted above of the evaporation of technique under stress in combat). The benefits of curved blades vs straight blades was argued at length in military circles in the late 19th and early 20th century - the length of the debate resulted at least partly from curved vs straight not making much difference, except in the cases of extreme curvature.

• Single-edged vs double-edged: like curvature, this often makes little difference. There are some cuts that can be done with a back edge, but most use the leading edge (and many single-edged weapons have a partial back-edge that allows such cuts anyway).

• Hand guard: simple sword guards, such as a basic disc guard or a straight cross, provide little protection. If the sword-wielder depends on such a guard for hand protection (rather than positioning of the hand(s) out of danger, or moving them out of danger), they can typically be hit in the hands fairly easily (perhaps not a big deal if good hand armour is being worn). Even a simple knuckle bow provides little passive protection. Partial baskets and full baskets provide good hand protection, but add to the weight and bulk of the weapon, and make it harder to wear everywhere and everyday.

Much of the difference between different types of swords due to the things noted above will simply not matter at all when it comes down to sword vs bow and arrow, and sword vs spear. They will also not matter as much on a crowded and chaotic battlefield. In a one-on-one duel, they will matter the most, and in those circumstances, one sees the development of types of swords to focus on one of those advantages (e.g., the rapier for length, and the smallsword for low weight and consequent speed).

what are some unique weapons from throughout history that aren’t relatively well known?

Flexible/chain/rope weapons are often ignored. Examples include flails, various Japanese chain weapons such as the manrikugusari and kusarigama, the Chinese rope dart, and more. Some of these are designed to get around shields and parries, or to provide reach (a weight on a long rope/chain) to attempt a decisive first blow against a swordsman or spearman. Others are designed as less-than-lethal police weapons.

Chinese martial arts provides some examples of unusual weapons, such as the hook sword, three-section staff, Emei piercers, and more. These were notably absent from the battlefield, and were perhaps not as effective as superweapons as their propaganda might suggest.

One often-ignored but effective weapon is the sling (the David vs Goliath type).

References:

Bambang Harsrinuksmo, Ensiklopedia Keris, Gramedia, 2004. [In Indonesian.]

Robert G. Hoyland and Brian Gilmour, Medieval Islamic swords and swordmaking, Gibb Memorial Trust, 2006. This presents al-Kindi's work on swords, with the original text, and English translation, and commentary.

George Silver, Paradoxes of Defense, 1599, http://www.pbm.com/~lindahl/paradoxes.html and see also Geroge Silver, Brief Instructions Upon My Paradoxes of Defence, 1898, http://www.pbm.com/~lindahl/brief.html

There are a few well developed and well known typologies that can be used to distinguish certain swords. Most are euro-centric, so I'll cover some of those here.

The Petersen typology is the most commonly used typology to identify types of "viking" era swords, those being norse swords from around 700CE to 900CE. It is covered in his book "De Norsk Vikingsverd". It is not the only viking sword typology - the geibig typology also exists, and covers some swords that Petersen does not, such as single edged designs. "Swords of the viking age" by Ian Pearce is a good resource on this.

The next typology, chronologically, is the Oakeshott typology, which classifies Medieval double edged swords from around 1000CE to roughly 1500CE. It is perhaps the most famous sword typology, and covers an incredibly vast range of designs. It is based on Ewart Oakeshotts book "Records Of the Medieval Sword".

AVB Norman made a typology of Rapier and sidesword hilts, which is very useful for classifying the vast range of variations in this type of sword.

Recently, James Elmslie wrote up his typology on single edged european swords, namely the Messer and Falchion. No published book yet, but you can find the typology online.

There is far more in the world of swords than just Europe, and each culture has its own unique designs. You'd need a library of at least 50-100 books to cover all the variants of swords through the ages. If you're interested in a particular region and time period, feel free to ask, as I have a large collection of original antique swords, and books on the matter

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