It should not be confused with 'schedulers' as in 'diarising' software which simply maintain a computerised version of your pocket diary. The server is the component which performs all of the scheduling and the running of tasks. The client is simply a front-end to the server to enable tasks to be configured on the server. Multiple clients are supported. The client and server can be run on the same machine, but more often, it is usual for the client to be on another machine and the communication between the client and server is done across a network - in other words, the client is used to administer the server remotely.
Communication operates on all standard network operating systems.