Heterosexuals are those people who are exclusively, sexually attracted to members of the opposite gender, as opposed to homosexuals or bisexuals.
Heterosexuals are the majority sexuality, comprising between 80 and 90 percent of the general population. Thus it is that heterosexuality is sometimes seen as the "default" sexual orientation. Occasionally, some people even incorrectly believe it to be the only sexual orientation.
It should be noted that heterosexuality can sometimes be an identification that the individual makes for themselves, and does not necessarily indicate behavior1.
For example, some cultures, such as Haiti, have homosexual activity as part of their spiritual activities or as acceptable behavior in their culture, without the participants identifying themselves as bisexual or homosexual.
See also: the Kinsey Scale.
1 Ok, so this is a little confusing. One doesn't always identify oneself as what one really is. Just like there are thousands of people out there who identify themselves as nice people, but their behavior clearly identifies them as jerks.