Counter-Strike: Source/Player models
When playing Counter-Strike: Source, you have a choice of how you appear. There are four different player models for each team, and any player can choose any of the four for his team. Knowing the different player models is important, so you can recognise who to shoot in an instant.
It should be noted that no model gives any advantage over the others; the differences are purely cosmetic (although it could be argued that some models blend into the background in certain maps better than others, and you might want to bear that in mind when choosing a model for a map).