Soldier
In Assassin's Creed II, there will be 5 enemies, as opposed to the original Assassin's Creed where you fight the same guard type over and over, with the exception of archers.
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 will cause a problem for some people, but it also makes them cumbersome.
There is the standard guard who are armed with a sword, and 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 when Ezio does a leap of faith, although Ezio can counter kill the seeker if that happens. They are armed with long spears or halberds which let them check haystacks, etc.