Death Guard are followers of Nurgle, the god of plague and disease. As such, they are a tough army, with interesting vehicles, and the ability to take a hit.
Death Watch are a low model count army that packs a punch. They're similar to the other Marines with some different options and powerful special abilities.
Grey Knights focus on smaller elite units, everyone is a psyker, which is rather unique, they are good at close combat, but do have some shooting as well.
Necrons shoot a lot and can go down to only stand right back up. Necrons have some good close combat units to help offset their slow and shooty nature.
Quantity over quality is how Orks operate. Orks are brutes who love close combat. The army works well as a horde but also have some fast, fragile vehicles.
Space Marines are the great generalists of the game, with no one thing that they really excel at, but they're good at everything, and are well-rounded.
