*puts nerd glasses on* Ahem..
They did explain it in SG-1. Ra was a Goa'uld hosted in an Asgard body, and later switched to a human slave when his Asgard body could no longer be kept alive. Granted the movie alien looks a bit different from the Asgards in the show (especially the teeth), but it's possible his Asgard body was kept that way with a sarcophagus since we know the Asgards have changed over the millenia due to their cloning reproduction process. As for the glowy effects when he gets angry, I'm assuming that's either artistic license from the director, or intimidation technology meant to scare the masses when Ra is pissed.
Another fun fact about the pilot episode of SG-1, it had full frontal nudity for Daniel Jackson's wife!
*keeps glasses on, fuck it*