The definition of a bisexual is someone who is emotionally and physically attracted to both genders. Bisexual however can be a term that covers many different sexualities.  What I call a True Bisexual is someone who is physically attracted to men and women, could fall in love with either gender, and they could end up marrying a man or a woman.

My defintions on some bisexuals:

70/30: A person who is mainly attracted to the opposite gender, occasionally they could swing to someone of the same gender, but they dont generally seek out the same sex.

"Beersexuals": A "bisexual" who kisses women (like in a club setting) to get the attention of men, is comfortable kissing a woman but isn't attracted to girls.

Bicurious: Someone who is exploring their feelings for how they feel twards each gender, and exploring their own sexuality.

A homosexual sorting through the process: Coming all the way out of the closet is hard and confusing! Society sends gays signals from the media that they should be straight, since the media aims at the larger population of straight people.  Homosexuals can be unsure at first. Also by saying they are bisexual the gay person can see how others feel about their same sex attraction and its a good halfway point before they come forward with others about being gay. 

Phew! I think that pretty much covers the different "bisexual" groups.  A true bisexual can get sort of a bad repuation because a gay person could fall for someone who they thought was bisexaul but isnt really romantically or physically attracted to them fully. Or a bisexual could date another bisexual but one of the bisexuals could be gay and going through the coming out phase and so the bisexual will help them through it.  A barsexual can frusterate or confuse gays, bisexuals, or straight people even!

True bisexuals can have people say things to them like "This is a phase" or "You're gay you will figure it out eventually" or "Pick a gender you can't be in the middle" or "You're just doing it for attention." But being bisexual can be a sexual preference you remain as for the rest of your life and you wont transition or change from at all.  A True Bisexual isn't confused they just like both genders.