A bisexual person can be sexually attracted, sexually active, and/or in love with people of either sex. Note that the attraction to both sexes is not always equal; a bisexual can be more attracted to people of one sex than the other.
Some studies, notably Sexual Behavior in the Human Male (1948) and Sexual Behavior in the Human Female (1953) by Dr. Alfred C. Kinsey, note that when asked to rate themselves on a continuum from completely heterosexual to completely homosexual, and when the individuals behavior as well as their identify is analyzed, the majority of people appear to be at least somewhat bisexual. Most people have some attraction to either sex, although usually one sex is preferred. According to Kinsey, therefore, only a minority (5-10%) can be considered fully heterosexual or homosexual; conversely, only an even smaller minority can be considered "fully" bisexual.