The practice of scheduling allows you to making things happen in a pre-determined amount of time. Scheduling is very different from ordering: ordering is a primarily technical consideration to ensure that the server synthesizes the sound in the right order; scheduling is a primarily musical consideration that allows you to control the perceived time that things happen.
SuperCollider's clocks have two main functions: they know what time it is, and they know what time things are supposed to happen.
There are three types of clocks, which each do slightly different things:
TempoClock: These clocks are aware of metre (time signature) changes, and have an adjustable tempo. They are to be used for scheduling musical events, and they run with a high priority.
SystemClock: This clock always runs in seconds. It can be used to schedule musical events, since it runs with a high priority. There is only one SystemClock.
AppClock: These clocks always run in seconds. They are to be used for graphic events and other non-musical things not discussed in this guide. These clocks do not run with a high priority, so they can be temporarily "side-lined" by a TempoClock or the SystemClock, if one of those has something to do urgently.
