People that are both romantically and sexually attracted to the opposite gender. So females are attracted to males/males are attracted to females.
This is obviously the most common sexual orientation in the world, due to biology. (You can only get offspring if a male and female have sex) and probably due to religious and social factors (it's a crime in some places to not be heterosexual, and many religions condemn homosexuality/bisexuality)
However, just because it's the most common, and "socially/biologically/religiously" corrct, doesn't necessarily mean homosexuality and bisexuality are wrong or "weird".
Everyone should have the choice and right to be what they want to be, including their sexual choice. It's in no one's place to judge or condemn people because of their sexuality. Just like it's wrong to judge people based on his/her race, sex or socio-economic class.
I'm a bisexual male, but prefers females to other males. (Bisexuality does not mean you are secretly gay, or like both genders equally)
So I am half heterosexual? lol.