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.

Counter-Strike.com
You can NOT view this value[tickrate] from the Netgraph (See “What is The Net_Graph 3” below). Server tickrate values are constant and do not fluctuate.

# Server tickrate = 100
# Server tickrate = 66
# Server tickrate = 33

The client's tickrate changes to match what is set on the server. This is why it's important that you change your client settings to match the server's settings. If your client settings are not set to match the server's settings, then you wont see the benefits of higher tickrates. See the section below on “What Should My Client Settings Be” to change your client settings to what they should be.