Service and a process

Clarifications

Essentially services are background processes. They either run all the time whnetger anyone is logged on or not, and or they are available for all logged on users, instead of having to run one instance for each.
There are a few rules for making a process a service, but the essential split in task manager is processes that have been registered as services on installing them.

Your Antivirus is a Process (running in the background and having a single function).

The 'Print' command is a Service - you click on 'Print' in any one of many utilities like M$ Word or whatever, then the Print Service goes off and finds all the processes necessary to produce a printed sheet of paper.