Most of what will be said in this doc is not Linux-specific, but should apply to most Unix systems
A process running on a Unix system can be managed using five numbers:
Process ID: a number defined by the system when the process starts. It is unique: you may have many xterms running but only one will have 123 as its PID .
Note:each process has a parent (except init, being The First) and keeps the parent's PID stored somewhere called PPID.
User ID: usually the UID of the user that started the process, it is used to manage permissions and accounting.
Priority: a number ranging from -20 to +20, negative numbers being reserved for the Superuser.
The lower a job's priority number, the more important it is, go figure... Use nice (renice) to define (modify) a process's priority when starting it (any time).