One common definition of an asexual is of a person who is not interested in sex. This definition is often used by people who are trying to explain asexuality in simple terms to the people around them. While sexual attraction is a nebulous clinical term and can be argued about almost endlessly, simply not being interested is harder to refute.
Some people who identify as asexual do not consider themselves to be uninterested in sex. They may be interested in sexual activities in order to please a romantic partner. There are also asexuals with an academic interest in the psychology or sociology of sex. Also, some people are not interested in sex, but might not be considered asexual; for example, a person who decided to be celibate for life might still experience the urge to have sex with another person, but ignore it. Some people have amended this definition to describe an asexual as a person who is not innately interested in having sex with anybody. But because innate interest is nearly as hard to define as sexual attraction, many people do not see the point in having this alternate definition.