In the classic doggy/doggystyle sex position, the woman kneels on all fours, facing down while the man penetrates her vaginally from behind. They both face the same direction. (This is not to be confused with dog play, though they can be combined.)
Although the man's crotch doesn't create any clitoral stimulation, the angle is good for more direct G-spot stimulation. Many couples enjoy the deep penetration this kind of rear entry sex position allows. It is easy for the man to set up a steady, rapid, deep thrusting. Both partners have a good range of movement and can each thrust as she can move back on to his penis.
He gets a good view of and easy access to her anus. This can make some women feel that it is an unflattering position which can either inhibit them or simply add to an excitable feeling of humiliation.
Anyone who has seen dogs mating can work out why it is called "doggy style."
Instead of being on all fours, the woman's shoulders are resting on the surface, making her torso lower and raising her bottom up. With her arms free, she can reach back and fondle his testicles or play with her clitoris.
The woman kneels upright.
The man stands by the bed (or whatever other similar-height furniture is handy) and enters the woman while she is on all fours with her feet hanging over the edge. This can be more comfortable for the man and allow him to wield a crop or paddle more easily while fucking her.
The woman stands and bends over sharply at the hips.
Whereas an inhibited Christian might feel that only the missionary position is acceptable, the ancient Taoists believed that humans could learn a lot about lovemaking from animals and therefore promoted doggy style rear entry position as the classic method of sexual intercourse.