Bisexuality is a sexual orientation. Bisexual (also bi) people are attracted to both men and women. Some bisexual people love men and women the same and some love one more than the other. A person's sexual orientation can range from heterosexual to only homosexual, but it can also fall somewhere in between. A person can be mostly heterosexual but have some homosexual feelings. Or a person can be mostly homosexual and have some heterosexual feelings. Some of the people who are somewhere between only homosexual and only heterosexual are bisexual. Where any person fits in this range is referred to as their sexual orientation.
In 1948, Alfred Kinsey published the Kinsey scale. The Kinsey scale shows that sexuality is a continuum, meaning it moves little by little from heterosexuality to homosexuality. On the Kinsey scale, a 0 is someone who is only heterosexual. A 6 is someone who is only homosexual. Someone who is equally homosexual and heterosexual (bisexual) is a 3.
In biology bisexual can mean an organism that has both male and female organs. This talks mostly about plants. Animals and people who have some male and female characteristics or organs are called hermaphrodites or intersexual.
- Rosario M.; Schrimshaw E.; Hunter J. & Braun L. 2006. Sexual identity development among lesbian, gay, and bisexual youths: Consistency and change over time. Journal of Sex Research. 43 (1): 46–58. 
- Sexual orientation and bisexuality FAQ's from the American Psychological Association
- James D. Weinrich, Sexual Landscapes: Why We Are What We Are, Why We Love Whom We Love, Charles Scribner's Sons, December 1987.