Prior and earlier can both be used in many cases, but not all. "I ate breakfast earlier." = "I ate breakfast earlier than now." "I ate breakfast early."= I ate breakfast at an early time. "I ate breakfast prior to starting out." - Note that prior needed some event to compare with it. Also, placement is different. I could say: "Prior to my graduation, I got married." It could also be the reverse order. I would say: "I got married earlier than my graduation." (Actually, you would seldom hear it put this way - more likely - "I got married before I graduated."

Something that happens before an event Is called prior