Definitions

Process: a sequence of instructions
It performs some useful activity
It may need to communicate with other processes
It may need to communicate with I/O devices
It may have some time constraints

Task: almost a process
Job: each activation of a task
In the real-time and embedded world we prefer "task"
A task is a sequence of operations with time constraints
Most tasks are periodic (once job every 50ms, for example)
Non-periodic tasks may be activated by events

Scheduler: the part of the OS that allocates the CPU to tasks
A real-time scheduler is quite different from a general-purpose on
The scheduler of a PC, or a server, must serve all processes fairly
An editor must respond quickly, which a compiler may lag
In control systems you want stuff to happen for sure
And within a maximum allowed delay