Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Nagini, Lord Voldmort's pet snake disguises herself as the old woman Bathilda Bagshot. She takes Harry upstairs and they hava some kind of conversation with hissing words, then she transforms into her snake self and attacks Harry, wrapping around him as if she is going to eat him. Harry hits her with a brick but she keeps on attacking in a storm of the kind of phony explosive hissing sounds we hear when we see every snake in a movie. Harry is almost lunch but then Hermione shows up and shoots her wand and the snake retreats. It comes back but someone shoots fire at it and Hermione and Harry turn into smoke or something and then fly out a window or something. Who really knows what's going on in these wand-wiggling movies? Not me.
In case you were wondering, the snake is not real - it's a fake computer-generated python.