Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Medusa shows up in a bunch of films. She is one of three Gorgons in Greek mythology - ugly women with live snakes coming out of their head instead of hair. If someone looks at a Gorgon, the snakes scare them so much they turn to stone. In the myths, after Perseus cut off Medusa's head, he uses it and its snakes as a weapon to kill people. Even though in myths she's ugly, in most movies (and halloween masks) Medusa looks like a hot chick who's just having a bad hair day. In this film Medusa is played by Uma Thurman who makes the snakey hairstyle look fabulous. I know that none of these are real snakes, but I enjoy watching Uma Thurman in tight black leather move her head around stiffly pretending that its covered with dozens of computer-generated snakes. Now that's acting. It's a nice touch having Percy (Perseus, get it) use the mirrored back of his iPhone to look at Medusa instead of the polished shield the real Perseus used.