Sexual intercourse is the pairing of opposite-sex (or hermaphroditic internal fertilization animals) for copulation. The primary biological purpose of sexual intercourse is the creation of offspring.
Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization. The two individuals may be of opposite sexes or hermaphroditic, as is the case with, for example, snails.
In humans, unlike most animals, sex may or may not be related to reproduction. In most cases people have sex for pleasure; this behaviour is also seen in some animal species, for example chimpanzees are known to have sex when the female is not fertile, presumably for pleasure, which in turn strengthens social bonds.