Bi-curious
According to some, the term bi-curious typically describes a straight person who is curious about being sexually intimate with a member of the same sex, but it can also refer to a homosexual who is curious about being sexually intimate with a member of the opposite sex.
For instance, if one is a heterosexual female and one is strongly curious about having sex with another female, one could consider herself bi-curious. Or as another example, a homosexual male could consider himself bi-curious if he has romantic fantasies about a female.
The term bi-curious is also used to specify a person with bisexual "tendencies" or desires. Bi-curiousity can be just a stage in a person's life where one is curious about another sexual orientation or a stage in life right before making a transition from one sexual orientation to another.
More precise explanation
While the definition above is good enough for some, a more academic definition with greater respect for the distinction between biology, gender identity, sexual orientation and sexual behavior would be as follows:
Bi-curious typically indicates an interest on the part of the person identified as such to retain a particular sexual orientation that involves attraction primarily to people belonging to one gender, while having an emerging attraction to people and/or activities that do not strictly conform to that person or others notion of that particular sexual orientation. It is most often used as a label for people who primarily identify as heterosexual, not bisexual, but who challenge some of the boundaries imposed by the labeler's notion of heterosexuality.
While it is, like bisexual, heterosexual, etc., a term of self-identity, bi-curious is also a term found more often than not in personal ads and in the marketing of sexually explicit dating services and literature to people who primarily identify as heterosexual. It is also used by peer support groups in order to reach out to people who are wrestling with personal sexual orientation issues. On rare occasions it is used by conservative religious groups to lure such individuals into forums where self-questioning and sexual experimentation are harshly discouraged.
Some consider bi-curious to be a trite label for its implication that one's sexual orientation is merely a behavioral choice that one can "try out". Others consider it to be a form of denial or homophobia, and that having an interest in more than one gender is by definition nothing less than bisexuality. Such complaints are often predicated on widely-held, but nonetheless contentious, beliefs about the nature of sexuality and gender — concepts that some feel are well-defined and constant, and that others feel are relatively fluid.
Identification as bi-curious or even bisexual doesn't necessarily require an interest in having sexual or even physical relations with more than one gender, just as celibacy or relative disinterest in sexual activity don't necessarily preclude identification as heterosexual or homosexual. The level of interest and desired degree of intimacy that is required in order to identify as bi-curious varies from person to person.
People identified as bi-curious may or may not strongly identify as male or female, may or may not strongly identify as heterosexual or homosexual, and their physical features and behavioral affectations may or may not be seen as being androgynous.
Like bisexual, the term bi-curious is sometimes applied to situations that do not fit easily into common definitions of heterosexuality and homosexuality. For some, bi-curiosity may be an aspect of pansexuality.