This amazing trailer gives a detailed guide to playing zombie panic source. Should definitely check the mod out.

In this mod, players play as either the survivors or zombies. At the start it's only one zombie player. When the zombie kills a survivor, that person becomes a zombie as well. The point is usually for the survivors to escape from the map and the point for the zombies is to kill all the survivors. Each side has benefits. Zombies regenerate health and have zombie vision. Survivors can usually outrun zombies and have weapons.

As long as you have a Source game (HL2, DOD:S, Left 4 Dead, etc.) you can directly download the mod off Steam.