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 only 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 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 intersexed.
Bisexuality Facts for Kids. Kiddle Encyclopedia.