there are two kinds of peanut butter - confusingly named "natural" and "conventional"
natural is just peanuts (and usually salt, tho you can get it without) - as luka said, the oil separates and you have to stir it the first few times you use the jar
conventional - i.e. Jif, Skippy, etc - uses hydrogenated vegetable oil as an emulsifier, and it's also usually full of unneeded sugar
idk the exact history - probably not hard to look up - but presumably "conventional" was developed as a convenience so people didn't have to stir
when I was a kid the Jif style was the default but nowadays you can get real peanut butter at any grocery store here, every big brand does both
natural peanut butter is a million times superior. Jif is like Nescafe compared to real coffee with the added detraction of being unhealthy.