What's the difference between 'modification' and 'corrective action' in an AD or SB?
A modification is a change to the airplane's physical configuration from the standpoint of fit/form/function.  Think Service Bulletin with a parts kit. Or an SB that has instructions for a physical rework of a part that makes it different from non-reworked parts, and the part is re-identified.

A corrective action is any action required by the AD to mitigate the risk associated with it.  Usually it's something less than a mod, like a inspection, change to a maintenance program, crew procedural change, de-escalation of a check or procedure's interval, an operational limitation, or any other action as required by the AD.

Terminating action is some activity that completely resolves the AD and restores the airplane's original airworthiness state, as opposed to something that temporarily mitigates risk so you keep on flying.  Incorporation of a Mod, or permanent incorporation of a inspection regime that previously wasn't mandatory.

On a side note, one thing about FAA ADs that makes them a total pain from the OEM's standpoint is that the FAA will not issue revisions to ADs like other authorities do.  If there is a change to the AD's terms they will only issue a new AD that supercedes the old one. Makes getting minor changes to ADs very difficult because it has to be worth issuing a new one.