Sexual arousal is the process and state of an animal being ready for sexual intercourse.
Human sexual arousal
Unlike most other animals, human beings of both sexes are potentially capable of sexual arousal throughout the year, and there is therefore no human mating season. Things that precipitate human sexual arousal are commonly known as turn-ons. Given the right stimulation, sexual arousal in humans will typically end in an orgasm, but may be pleasurable for its own sake, even in the absence of an orgasm.
Causes of human sexual arousal
Signs of possible human sexual arousal
Human sexual response cycle
During the 1950s and 1960s, William H. Masters and Virginia E. Johnson conducted many important studies within the field of human sexuality. In 1966, the two released a book, Human Sexual Response, detailing four stages of physiological changes in humans during sexual stimulation. These phases, in order of their occurrence, are excitement, plateau, orgasmic, and resolution.
See human sexual response cycle.
Homophobia and sexual arousal
Psychoanalytic theory has long held that homophobia is the result of repressed homosexual desires. A study showed that more homophobic heterosexual men (80%) showed signs of arousal from being shown images of homosexual sex than non-homophobic heterosexual men (34%)  (http://www.apa.org/releases/homophob.html). The two groups were, however, aroused to the same degree by heterosexual imagery and lesbian imagery. The authors noted as a competing explanation that anxiety also produces arousal and might be responsible for the difference, so further research should test the two competing explanations.
Genital procedures and sexual arousal
Scientific data shows that male circumcision causes neurological changes in the human brain, which is suggested to lower sexual excitability (Mackey, 1997).  (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9446966&dopt=Abstract) Another study suggested that arousal of a man's female partner is also impacted.  (http://www.nzma.org.nz/journal/116-1181/595/)
Sexual arousal in other animals
It is not completely understood how other animals relate sexually, but current research studies suggest that animals, like humans, enjoy sexual relations. This is especially noted in dogs, dolphins, and bonobos.