"Feminism is the radical notion that women are human beings."
--Cheris Kramerae, author of A Feminist Dictionary, 1996
You've probably heard lots of things about feminists. They're man-hating lesbians who like to burn their bras in public. Or, bitter women who don't shave their armpits and despise sex. AND, if you listen to Pat Robertson, feminists are encouraged to kill their own children and overthrow the government.
If you haven't guessed it already, all of the above--ranging from the silly to the outright ridiculous--are simply stereotypes. The actual definition of a feminist (according to the American Heritage Dictionary) is someone, male or female, who believes in social, political and economic equality between the sexes. Basically, feminists believe that women and men should be treated the same and given the same rights.
Now that you know the definition of this f-word, here's a little background info. Even though the term was developed in France (feminisme) in the mid-1800s, it didn't become popular until the 1960s. By the 1960s, the women's movement was well underway and women all over were campaigning for equal rights across the board--from eliminating job discrimination to gaining reproductive rights and political representation in the government. Previously, these women's groups had used the term "women's liberationist" to define themselves, but as it began to lose popularity they embraced the term "feminist."
So how did this word come to have such negative connotations? Like most things that are challenged, the previous, male-dominated power structure felt threatened. Change can be scary; and now women were demanding equal pay for equal work or the ability to make choices about their own bodies. Women were rightfully angry and did some very radical things (though no bras were burned, contrary to popular belief). It is much easier for people to put down feminism than admit that society must change its ways.
Also, many people tended to (and still do) think that since the term "feminism" has "fem" in it, a feminist must be someone who wishes to raise women above men. But if you look to the actual definition of a feminist, it's simply about creating an even playing field for men and women.
And you cannot forget the people or language used that encourages these negative connotations. The term feminazi (feminist + nazi) has been used to criticize feminists, purporting the stereotype that they have a Nazi-esque hatred for men as well as believing that women are superior to men. Therefore, a cycle of anti-feminist discourse is always playing.
For more info on feminism, check out:
Feminism on Wikipedia
Feminist.com
The National Organization for Women