What is the difference between regular aspirin and buffered aspirin

Aspirin is acidic. A buffer is a substance that balances acidity to make the drug less agressive to the stomach lining.
The buffer used is calcium carbonate, which neutralises acid.

Aspirin, acetylsalacylic acid, is a weak acid. It does not protonate well at the low pH of the stomach. For this reason, the compressed tablet tends to maintain its shape & sit on the stomach lining, undissolved, where it can cause irritation.

By combining an amphoteric base (the buffered part of buffered aspirin), the local pH is swung toward a more basic condition & the aspirin can dissolve more easily & disperse quicker.

There r other tricks of the trade to accomplish the same thing, but some r more expensive, like adding exipients or putting the asprin in a microencapsulated form.

buffers=sucrose or other fillers to make the tiny amount of medicine more gentle on the stomach. About 90%+ is not medicine, anyway.