Bisexuality is a sexual orientation. Bisexual (also bi) people are sexually 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 popular myth within the LGBT community is that bisexual people can only be attracted to cis people (people who identify with their assigned birth gender), and/or males and females. This is incorrect. Bisexual people are fully able to feel attraction toward all genders, including non-binary genders, and all binary genders.
Some bisexual people have preferences to one or more genders, however some may not. Both kinds of attraction are entirely valid and accepted by the bisexual community, as bisexuality is fluid and is a different experience for every bisexual person.
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.