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I think it's more important to learn how the deamons handle these IDs than the IDs themselves.
I have read that all linux processes have IDs
And also the deamon may need all these IDs to start a process in an environnement
Each process has its own ID number. That's how they are identified. If you run 'top', that's PID in the first column. A process can have a parent process - a process it was started from. The parent process usually waits for the children process to finish. Parent process is also PID.
Group ID you mean is, I guess, connected to user names. In fact, the usernames doesn't matter. Every user name has a number (user ID, UID) asociated with it. Every group has an ID, too (group ID,GID). File owners are identified using numbers, not names. So if you delete an user and then create new with the same name, new user won't have access to files owned by the old user (it may happen that the new user will get the same UID as the old user, but it's rare).