Asexuality is a term for people who do not experience sexual attraction.
Little research has been done into asexuality, but a UK survey of sexuality indicated that 1% of adults had never felt sexually attracted to anyone at all. (PubMed) A study of rams (referenced here) also concluded that 2% to 3% of the rams were asexual.
There is debate over whether to see asexuality as one of four or more orientations (homosexual, heterosexual, bisexual/pansexual, asexual) or as one of two (sexual and asexual, with gender preference being measured along a different axis). Relevant thread
There are also some classifications that can be made within the spectrum of asexuality, based on [romantic attraction] (some asexuals desire romance with one or more genders, and some don't) and [sex drive] (some asexuals experience sexual arousal, but without the desire to express it with another person). At one time, AVEN sought to sort asexuals into four categories, based on these criteria:
Type A asexuals, who experience sex drive, but no attraction; Type B asexuals, who experience romantic or other forms of attraction but do not have sex drives; Type C asexuals, who experience sex drive and romantic or other forms of attraction, but do not see them as linked; Type D asexuals, who experience neither.
This classification system was retired, because not all asexuals felt comfortable putting themselves in one of the four categories. However, some asexuals still find it useful to reference the concepts of type A, B, C, or D at times. Relevant thread