'Asexuality (or nonsexuality) is the lack of sexual attraction to anyone, or low or absent interest in sexual activity,' according to Wikipedia.
But, as always, things are not so clear cut. Take attraction, for example. Some asexuals feel very attracted to others, and want to be in relationships, but just without it getting sexual. Other asexuals don’t feel attraction or want any type of relationship.
When it comes to arousal, it's not straightforward either. Some asexuals get aroused regularly, without the arousal being directly related to wanting sex with somebody else. Others masturbate occasionally, while some feel no arousal at all.
If someone decides not to have sex for personal reasons, or due to religious and cultural beliefs, he or she is celibate. Being celibate is different from being asexual, because it’s a choice. People who are celibate do feel sexual desire and sexual attraction towards others, but people who are asexual don’t.
Not all asexuals are actually celibate. Some people who are asexual do have sex. Many asexuals have partners and relationships, and some have sex with their partners – they might do it just to please their partner, or because they want to have a child.
Nevertheless, while some asexuals do have occasional sex, others are sex-repulsed. That means that sex can make them very, very uncomfortable, and even the thought of sex can make them feel literally sick.
Whether or not asexuality is a sexual orientation is still debated in the scientific community. What is for sure though, is that asexuals can’t change. Don’t expect them to start feeling sexual desire when they ‘meet the right person’. People who are asexual are born that way, and there is no way of ‘fixing’ or ‘curing’ them. It would be insulting to imply that you could.
There is no cause for asexuality. People are just born asexual. It’s neither a mental nor a physical illness. Asexuals haven’t been sexually or otherwise abused or are closet homosexuals. They don’t choose to be asexual.
Many people who are asexual are perfectly happy the way they are. Being asexual doesn’t change anything about a person’s behavior, other than their sexual behavior. And there’s no way of telling whether or not someone is asexual by looking at them.
Whether you consider yourself to be asexual all depends on you. In the end, asexuality is a personal orientation. Only you can decide whether you’re asexual or not. There aren’t fixed categories or tests that tell you that you are asexual. Of course there are certain signs, like never having felt sexually attracted to anyone, but in the end, it’s up to you to decide what to call yourself.