"How to Train Your Dragon" the movie is a completely different beast to the series of books of the same name. In fact, with a few name changes they would be almost unrecognisable as related. The plots are substantially different, though the movie could possibly operate as a much earlier prequel to the books if the names were changed.
There are characters who appear in the movie but not in the books vice versa. There are also characters who are substantially different in the movie to their original incarnations.
Characters who appear for the first time in the movie
Astrid Hofferson is Hiccup's love interest and primary sidekick. She essentially replaces both Camicazi and Fishlegs from the books, although Fishlegs does appear in the movie.
Ruffnut is Tuffnut's fraternal twin and provides the welcome addition of another female character in the movie. Initially she is not fond of Hiccup, but as his skills improve she becomes quite a fan and even begins to flirt with him, until Astrid stops her.
Important characters who appear in the books but not the movies
There are actually a lot of characters who only appear in the books, as you would expect given there are several books and only one movie. Some of the important ones are:
Camicazi is a Bog Burgler and doesn't appear until the third book, How to Speak Dragonese, when she is captured along with Hiccup and Fishlegs by the Romans, and helps Hiccup engineer the final part of their escape. She then becomes Hiccups's main helpmate in the fourth book, How to Cheat a Dragon's Curse, as Fishlegs is at home sick. She is replaced by Astrid as the main female character in the movie.
Dogs Breath the Duhbrain is Snotlout's main offsider in the books but doesn't appear in the movie.
Characters who are changed for the movie
Fishlegs is hiccups best friend in the books. He is weedy and asthmatic and just as unpopular with the bigger boys as Hiccup. In the movie he is chubbier and bigger than Hiccup and is accepted okay by the other teenagers. He is just a minor character, effectively replaced by Astrid as Hiccups's main offsider.
Toothless is Hiccup's hunting dragon in the books, which means he would not be big enough to ride even if he were full size. As it is he is the smallest hunting dragon anyone has ever seen. He is an emerald green Garden Dragon (as in 'garden variety'). In the movie he is the rare Night Fury, a jet black dragon who is big enough to carry Hiccup and Astrid together on his back while flying.