Bisexuals are people who are attracted fairly equally to members of both
genders, as opposed to
heterosexuals or
homosexuals.
Until the last few years, bisexuality has been the small, hidden, persecuted minority of sexual identifications. Hounded by mainstream society for same-sex relationships, hated by homosexuals for "fence sitting" and "not being able to make up their minds" and indiscriminate promiscuity. Bisexuals are still struggling against these prejudices, but are starting to make their voices heard.
See also: the Kinsey scale