A person's sexual orientation indicates a primary group of people that the person finds sexually interesting, attractive or desirable. The groups are divided by sex (biological gender) and/or gender (social/mental gender or gender identity). The five following orientations are the most covered and used: heterosexuality, homosexuality, bisexuality, pansexuality and asexuality.
Heterosexuality is considered to be the norm in today's world and the general usage usually assumes that there are two sexes/genders. A heterosexual woman would thus find men sexually interesting. The prefix "hetero-" comes from Greek and means "different".
Homosexuality means that a person is attracted to people who are of the same gender/sex as the person is. The prefix "homo-" comes from Greek and means "same".
Bisexuality also assumes that there are two sexes/genders. The definition is that a bisexual person can be attracted to both men and women – sex/gender doesn't or shouldn't matter. The prefix "bi-" comes from Latin (and other languages) and means "two".
Pansexuality is in theory similar to bisexuality, but it does not assume that there are only two sexes/genders – those who are intersexed or transgendered are not excluded from this sexual orientation, as far as literal wording goes. The prefix "pan-" comes from Greek and means "all".
Asexuality as an orientation means that the person has no sexual orientation, or that it simply is not directed towards others. However, this doesn't necessarily mean that the person would not want sexual intimacy or relationships, it is simply a lack of an orientation. The prefix "a-" comes from Greek and expresses negation.
Sexual orientation is often more complex than just dividing people into different sex/gender categories. Very likely not all people of the target sex/gender would be even potential partners – most people wouldn't for example start dating a person whose personal traits (such as essential personality characteristics, outward appearance, general intelligence or social class) are very displeasing to them. So, more accurately a person's target group could be put down for example like this: "lower-class, not fanatically religious men raised in general western culture, having long, blonde hair and a steady job".
It is certainly possible to overstep the set parameters for a potential partner. If the person seems otherwise very suitable, save for the steady job part, quite many people would probably make do and not care about the lack of a job. Similarly, some people can even overlook sex/gender of an otherwise perfect potential partner, even though usually (particularly for heterosexuals and homosexuals) sex/gender is the most important factor. Thus, sexual orientation is rarely set in stone, though most people most likely don't act upon attractions that are inconsistent with their sexual orientation.
Sexual orientation can be fairly fluid and change through a person's life. Then again, for some people it isn't so. It seems most often a sexual orientation isn't something a person chooses, as it just comes naturally to them. Again, some people can and do choose their orientations; for example a pansexual person who finds no problem in conforming to a heteronormative society.