This is from the help:

trigger
Record-level event code that runs after an insert, update, or delete. Different actions can be attached to the different events. Triggers run last, after rules, and do not run during buffered updates unless TABLEUPDATE( ) is issued. They are most often used for cross-table integrity.

>Like, on an update, does the trigger get fired for the current field every field, once or every time?

You can get to triggers from the MODI STRU window. The first ex. that comes to mind of using triggers is for cascading deletes.