What is the difference between tripe and offal?
Offal is a general term for any parts of the animal's innards which are not the conventional cuts from the muscle and bone.  It would vary by cuisine, but could include liver, kidneys, heart, brains, stomach, or tongue.  Those particularly fussy about what they eat might even extend it to things like tail and neck, although most wouldn't regard these as offal per se.

Tripe, on the other hand, refers specifically to the stomach. It's usually washed, and often bleached, then slow cooked, although again, the preparation varies by region.  Common tripe dishes come from the stomachs of cow and sheep, and despite falling out of favour as a cheap alternative to regular meat, it's still quite popular in some parts of Europe.