To answer your question of what is it - Zombie Mod is basically this:
At the start of the round you spawn as either CT or T. You then either buy guns or select them from a weapons/guns mod, depending on the server settings, and, also depending on the map, find a place to barricade yourself or do a marathon around the map to a escape point. Most ZM (Zombie Mod) servers allow you to use the props of the map, such as vending machines, couches, tables, etc, as barricades. Some even have hidden areas.
A little while after the round starts, someone turns into a Zombie and must infest the alive population, so to speak, and only spawns with a knife. However, there are also 'Zombie Classes', that have different perks, such as more speed, higher jump, more health, etc. Some classes also sacrifice these attributes for more input into another perk, such as one class having the most speed, but less of, for example, health.
With a large number of players its a really fun gametype..
If I recall, eGO had a ZM server way back in the earlier days of CSS, but I may be wrong.