A buckskin is a bay + cream. They CAN have a dorsal stripe, but it's just called countershading and is basically like when you overlap when writing with a marker and that spot becomes darker. Buckskins are also a very gold colour and creamier.
A "dun" is technically a bay + dun. They have dun factors that include a very defined dorsal stripe, zebra barring on legs, shoulder barring on the withers and neck and cobwebbing on the face. The colour is kind of a "dingier" shade of buckskin. It's not as golden and is kind of..flatter?
You can have a buckskin + dun. It's the golden shade with dun factors.
If you want to tell them apart fast, just look at their legs. A dun will have zebra barring, a buckskin won't.
I'll go try to find one of the many threads discussing the differences