@scotty I'm unsure if this is a joke question or not, but I'll answer it regardless haha.
Shin Megami Tensei (previously simply called Megami Tensei) is the mainline franchise from Atlus. There are many spin-offs, the most popular of which is Persona. The Persona series used to carry the Shin Megami Tensei moniker in its name but was later dropped.
Their main differences are the themes and subject matter in each games' story. SMT is generally regarded to have a more mature, dark, and serious tone. While Persona is on the lighthearted side. Most of the demons/personas used are found in both games along with accessories like items being shared between the two.
There are of course plenty more similarities and differences but these are off the top of my head.