What is a tick?

Simply put, a 'tick' deals with the smallest amount of time the game engine deals with – it is how many discrete steps the game can move in one second. With a higher tick rate, game physics can appear smoother or more realistic, but this comes at a heavy cost of processing power.

In another sense, it's a count of how many 'samples' you take from a (theoretical) real-life scenario. The more samples you take, the less jumpy the data are.

The server's tickrate is the number of game frames that the server processes in a single second. This is sometimes also referred to as the server FPS, which is confusing as the server still has the max_fps command.

Every game frame, the game calls into an entity's GameFrame method (if it has one) for any game logic processing. For projectiles, players, doors, etc... this includes physics processing. The game also does internal logic at this time... and any Valve Server Plugins (VSPs) such as MetaMod:Source and SourceMod can also hook have their own GameFrame calls (for SourceMod, this is used for SourceMod timers and any plugins that have OnGameFrame hooks).

As you may have guessed, the higher you set the tickrate, the more CPU power the game will use.

It doesn't always have to be a power of 2. In fact, some CS:GO server owners swear by 102 / 102.4 tick instead of the default 64 and 128 tickrates. They claim it makes the server's performance more consistent and reduces CPU spikes.

However, this may have changed as Valve has updated the game.

Keep in mind that setting the tickrate higher alone won't improve players experience. You also need to make sure your rates are set properly for the amount of bandwidth you have available.

As for Valve's other games, a lot of them use locked tickrates:

• 30 game frames per second
  • Left 4 Dead
  • Left 4 Dead 2
• 66 game frames per second
  • Day of Defeat: Source
  • Counter-Strike: Source
  • Team Fortress 2

The 66 game frames per second games were not always locked to 66. Valve changed this some time back after making changes to the game code. These changes made some games (TF2 in particular) much more sensitive to tickrate changes... setting it too high or too low changed the speed at which doors would open/close and caused issues.

From a Counter-Strike Forum:

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.

In one tick the game engine processes all actions (whether from a user or from an AI or other internal process) and updates the state and location of all objects in the world accordingly. The rendering engine takes all the states and locations and displays the objects onto your screen using on your location in the world. For a standalone game, there is normally a render-per-tick. For an online game, the relationship depends on the game - eg. your World of Warcraft client receives state and location information from the server and your local machine simply renders the objects at its own pace.