If you click on the Processes tabs you will be able to see existing processes and create new ones.
You can then call one process from another process.
For example if you created an approval process based on a table thats called Approvals for the sake of the example. That process could send an email to the approver in the Approver column of the row asking him to go to the app and mark the row as approved.
No you can use this approval process in many other processes. If you had an order process it could call the approval process in order to approve an order. Or if you had Job process you could call the approval process to get the job approved.
Processes are just a way to write logic that can be reused.
Hope this helps.