In WWI and WWII it seems that on any given day there was a continuous line, for the most part, where one side had one combatant and the other side had the other. I know the line wasn't perfect, but it was solid enough that in books on the topic like Barbarosa by Alan Clark he has maps showing the front line of the day that, in the Eastern Front of WWII, go from the Baltic to the Black Sea.
In earlier wars like the American Civil War or Revolution, it seems that the lines were more de jure and armies might travel around in enemy territory pillaging until they were finally challenged by the enemy army.
Is my idea of these two types of lines correct for the most part? If so when did this change occur and why?
Entire books could be (and have been) written on this topic, so my answer is by necessity pathetically brief. However I'll try to hit all the relevant points.
"Front lines" as you describe them were a mathematical inevitability of the technological and social changes of 20th century warfare. The biggest contributing factors include:
Nationalized conscription: Countries could now field armies that consisted of hundreds of thousands or even millions of soldiers. Furthermore technological advances meant that the logistical requirements of these armies could actually be fulfilled. However the size of the terrain these soldiers fought over had not changed. In short, they were able to pack more bodies into the same amount of terrain. This meant that the concept of a "front line" was suddenly feasible (even inevitable), because they actually had the numbers to turn it into a reality. The "front lines" of a medieval battlefield might consist of a couple hundred/thousand soldiers and only be a few hundred meters long. The "front lines" of a World War II battlefield could consist of millions of soldiers, and be hundreds of miles long.
Logistical capacity: This is sort of the reverse side of nationalized conscription. If you have armies that consist of millions of soldiers, the logistics to keep those soldiers supplied becomes extremely important. Protecting your own logistics, and disrupting the enemy's, becomes a vital goal. In order to do this, you create a "front line" that, even at the weakest points, gives some warning of an enemy advance. The enemy likewise does this. You end up with two opposing "front lines".
Advances in weapon technology: A soldier with a sword or spear can project a sphere of influence of perhaps 1-2 meters around him. In order to protect that soldier's flanks, you need another soldier about that distance away next to him. A soldier with a rifle, however, can project a sphere of influence hundreds of meters in any direction. In order to protect that soldier's flanks, you can place them much further away and still have them in a position where they can mutually support each other. Indeed, with heavy artillery, having those soldiers spaced out isn't just possible but ideal. Magnify this by thousands/millions of soldiers, add in crew-served weapons, artillery, aircraft, armor, etc., and it's easy to see how quickly a "front line" can be packed.
On a final note, it's very important to remember that "front lines" are a concept. Textbooks and history books include maps with tidy "front lines" to aid in our understanding of what occurred in a battle or war. However front lines in reality are hardly perfectly ordered. Gaps occur, mistakes are made, humans are humans, terrain doesn't cooperate with battle plans, etc., etc., and that's before the fighting even takes place.