Directly from Valve:
During each tick, the server processes incoming user commands, runs a physical simulation step, checks the game rules, and updates all object states. After simulating a tick, the server decides if any client needs a world update and takes a snapshot of the current world state if necessary. A higher tickrate increases the simulation precision, but also requires more CPU power and available bandwidth on both server and client.

So what that basically means is that 66 tick is ideal for servers that have more than 20 players in it. 100 tick is more for 20 players or less otherwise there will be some lag, unless the server has a good CPU. The player's computer specs also play a part in it. If the player's computer can't handle 100 tick despite the server being able to, then the player will still lag.

This is just here for anyone who is curious about the difference or wants to know why so many people cry about what a server's rates are.

With the new CS:S update, Tick Rates no longer exist!