A feminist is a person who believes that there is still a need for social change in the treatment of women. There are many different kids of feminism.... Marxist feminism, radical feminism, liberal feminism, psychoanalytical feminism... etc. Liberal feminism is the most common among white middle class women who simple believe in equal rights for women. However, feminism is not only about the legal rights of women, but the social attitudes about women as well.
One example is the epidemic of eating disorders among young women who are influenced by society to stay thin. Many feminists believe that this stems from the male desire (conscious or subconscious) to keep women small and vulnerable.
There are many issues which need to be addressed. Feminists also believe that males are oppressed by social standards they are raised with. For example, it is socially unacceptably for men to cry in public, whereas it's perfectly normal for women. These are social standards that limit the full potential of both genders. Men who are threatened by feminists should learn more about feminism before they insult is, as feminism really has nothing to do with the concept of hating men. We believe that men are oppressed as well. There are 'male studies' courses popping up in universities all over the world. Women just happened to think of it first.
If a man really takes the time to learn what feminism is, he will probably consider himself a feminist as well.