Warrior is a creature type.
The warrior creature type has always been around. However, since Wizards of the Coast adopted the new race/class model it has become the default creature type for any creature that is not a soldier and fights in wars. The type is most found on red, green, and black cards, while white cards tend to be soldiers (blue creatures are less eager to go into battle so they don't have either of these types as much).
In Lorwyn block, it was one of the main creature types and it got some mechanical support.