The Brotherhood of Mutants, originally called the Brotherhood of Evil Mutants, is a group of supervillains from Marvel Comics who fight against the X-Men. Their leader is the supervillain Magneto. Most members are mutant whose origin story involves being treated very badly by normal humans and who want revenge.
In the 2000 movie X-Men, Magneto tells the mutant-hating Robert Kelly “My dear Senator, we aren’t here to kill you, we’re here to welcome you to the Brotherhood of Mutants”. Within that movie, Mystique, a naturally blue shape-shifter who is in love with Magneto, Sabretooth, who is strong and has fangs and Toad, a mutant with the powers of a toad. In the sequel, X2, an X-Man named Pyro who can control fire leaves the X-Men to join Magneto and Mystique and they become the new Brotherhood of Mutants.
In the next movie X-Men:The Last Stand, Magneto frees a number of dangerous mutants captured by the government and makes them part of the new Brotherhood of Mutants. They include Juggernaut, who is very strong and cannot be stopped once he starts moving, Gemini who can clone himself and once robbed six banks at the same time, the Phoenix, a female X-Man once called Jean Grey a male mutant with the powers of a porcupine, a female mutant who can sense other mutants and what their powers are and Pyro.