Lead:
Self-explanatory. The role an entire story pivots around. Katniss, Liz Lemon, Bella Swan, etc.
By the way, you don’t technically need to have lines to be a Principal. In my project one of the supporting characters is silent. That doesn’t mean that she was billed as an extra. She is still a principal player and as far as the awards system goes she would be listed as a supporting actor.
So there you have it. I hope that clarifies things.