Could someone please explain what all the titles mean? I know the basic european ones, Baron, Count, Duke, King, Emperor. But is the russian tzar the same as emperor(in swedish emperor and tzar kind of has the same name, they both originate from Caesar) and in that case did old russia have multiple kings under the tzar?
This varies a lot depending on where in the world you are, what your religion is and when it is. Some countries have extensive ranks of nobility, othes have only a few. Some were there from the beginning, others have been added at different times.
The original nobility in Sweden consisted of Sven (Esquire) and Riddare (Knight). The Knight had to be knighted by the King - in the late 1400s, after Sten Gustavsson (Sture) had won the battle of Brunkeberg 1471, he was the only Knight in Sweden, as there were no King to knight anyone. The title of Hertig (Duke) was reserved for the royal family then and still is. Friherre (Baron) and Greve (Count) was added at the coronation of Erik XIV 1561.
Sweden
Kung (King).
Hertig (Duke, reserved for members of the royal family).
Greve (Count, created 1561).
Friherre (Baron, created 1561).
Riddare (Knight, have fallen out of use since 163)2.
Sven (Esquire, have been replaced with untitled nobility).
Germany
Kaiser (Emperor).
König (King).
Kurfurst (Prince-elector, a person ruling his own sovereign realm insie the Holy Roman Empire and being an elector for the Imperial Throne).
Furst (Prince).
Erzherzog (Archduke - in some cases the Erzherzog is on the same level as a Kurfurst, especially the Erzherzog of Austria, before Austria became an Empire of its own after the dissolution of the Holy Roman Empire during the Napoloenic War).
Herzog (Duke).
Markgraf (Marquis).
Graf (Count).
Freiherr (Baron).
Ritter (Knight).
Junker (Esquire).
British
King.
Duke.
Marquis.
Earl (Count).
Baron.
Baronet.
Knight.
Esquire.
Russian (including most orthodox slaviv-speaking countries)
Tsar (Emperor).
Velikiy Knyaz (Grand Duke/Grand Prince, reserved for the rulers and their immediate family).
Knyaz (Duke).
Graf (Count, introduced in Russia by Peter the Great)
Boyar (Baron, seem to have been replaced by Baron by Peter the Great).
To answer your initial question, I would argue that the equivalent of the Caliph in the Christian world was the Byzantine Emperor, or Basileus. Of course, this was challenged throughout the medieval period by the West (especially the Carolingians and the HRE), but even during the time of the Crusades (especially the 1st Crusade), many Crusaders seem to have expected the Byzantine Emperor Alexios I Komnenos, as lord of Christendom, to lead the forces of the Crusade against the Muslims and were extremely disappointed when he didn't.
It is also worthy to note that within the Empire, the Emperor was not only a political/military leader, but also often was the religious leader who could override even the Patriarch of Constantinople, and stylized himself as not only the leader of all of Christendom, but also God's Vice-regent on Earth.
I should note that Russian titles post 18th century were honorary. Peter the Great abolished the traditional nobility's power structure and replaced it with a (theoretically) meritocratic system based on the Table of Ranks. Everybody started out at the bottom and worked their way up.
(unless you got high enough, in which case you earned yourself hereditary nobility.)
The Wikipedia article gives you a good idea of its structure: http://en.wikipedia.org/wiki/Table_of_Ranks
As a fun fact, Lenin's father earned his family nobility, meaning that Lenin was actually a "High Excellency."
(EDIT: I misclicked somewhere, this was meant to be a reply to Vonadler's post)