A process is a set of interrelated actions and activities that are performed to achieve a pre-specified set of results. Processes have inputs, tools and techniques, and outputs.
The PMBOK® Guide (6th Edition) defines 49 processes covering the 10 knowledge areas. These are shown on page 25 of the PMBOK® Guide which is the most important page in the entire document. It should be understood and memorized as it is crucial to passing the exam. Page 25 categorises the processes under the Project Management Process Groups which are described on page 23.
Many PMP® exam questions describe an action and then ask which Project Management Process Group is falls under. An output from one process invariably forms the input to another related process.