Soldier
Assassin's Creed
During the course of the first Assassin's Creed, there are two types of guards: Soldiers and Archers. The archers are mostly the same, but the soldiers vary on skill, which can be identified by their helmets. The shinier the helmet, the stronger the guard.
Assassin's Creed II
In Assassin's Creed II, there will be five enemies, as opposed to the first game where you fight the same guard type over and over.
One of the new guards is the "Agile", which, as the name suggests is a very fast, lightly armored enemy. They also have the ability to climb and run around rooftops like Ezio, leading to chase sequences.
Next, there is the archer, which is a guard armed with a bow and a sword (for close combat).
There is also the brute, who is a heavily armored and equipped (weapons such as maces and axes), and rather slow guard type. Their armor makes them tough to bring down, but also makes them cumbersome and slow. They are not very intelligent, and won't be able to tell Ezio from anyone else in a crowd. A good way to make short work of them is dodge their attack and attack before they get the chance to defend.
There is the standard guard who is armed with a sword. They aren't particularly fast, strong, or smart.
Finally there is the Seeker, who is often called "paranoid"; they are a special guard type, because they have the ability to check haystacks and other hiding places, although Ezio can counter kill the seeker if that happens. They are armed with long spears or halberds, giving them an increased reach in combat. The best way to kill them is to disarm them and finish them off with their own weapon.