Bisexuals are people who are sexually attracted to members of both genders, specifically where they actively engage in, or would consider engaging in, sexual activities with either gender. They may be male or female.
Sometimes bisexuality is jokingly described as getting "the best of both worlds".
In inclination, many people are attracted to both genders to some degree although far fewer people might ever do something about it. Many people who have some level of attraction for both genders are reticent about admitting it; reasons for this include shyness, social pressure and because they would not welcome advances from both genders.
Anne Rice novels "normalise" bisexuality, especially the Sleeping Beauty trilogy.
A common euphemism for bisexual is AC/DC. In electrical engineering, AC and DC are alternating and direct current, representing hetero- and homosexuality.