My question is what is the difference between Prilosec and Pepcid?

Mostly the cost. Pepcid is in a group called H2 inhibitors and prilosec is in the group called proton pump inhibitors. They both block the production and use of stomach acid, in slightly different ways. For some pepcid doesn't work as well as prilosec, but for many it does. The H2 inhibitors are the older of the meds.