A process is a binary program running continuously on a computer (until terminated) which is managed by the operating system. A particular process may run multiple execution threads, but the flow of logic is generally directed by the main processes' thread.
The process executes on the CPU when the operating system's process scheduler determines that it should run. There are a wide variety of methods and algorithms used in operating systems to determine when to execute the next group of instructions in a particular process.
These schedules differ from operating system to operating system (or even in different versions). For example, the Linux kernel's process scheduler was changed from version 2.4 to 2.6.