Broadly speaking, the term sexual intercourse may refer to any sexual activities between people, but is commonly confined to the meaning of coitus or insertion of the penis into the vagina. Unlike some other sexual activities, sexual intercourse can therefore only be enjoyed by a mixed-sex couple (a man and a woman). Sexual intercourse may be enjoyed in a wide variety of positions, although various positions have been designated by various societies as "proper" and "improper". Sexual intercourse and masturbation are the two commonest human sexual activities.
Unlike some other sexual activities, sexual intercourse itself has rarely been tabooed on religious grounds (there have been exceptions) or by government authorities. Indeed, in some societies coitus has been the only "acceptable" sexual activity. On the other hand, relatively strict designations of "appropriate" and "inappropriate" sexual intercourse (wrong person, wrong time, wrong position, etc.) have been almost universal in human societies.
Sexual intercourse with a person without their legal consent (because they have been coerced by force, because they are too young to give informed legal consent, because they are intoxicated, etc.) is called rape and is a serious crime in most jurisdictions.
Heterosexual sexual intercourse (coitus) is the basic reproductive method of Homo sapiens as of all mammals and should always be considered likely to result in pregnancy unless adequate contraceptive (birth control) measures are in force. Coitus interruptus, or "withdrawal" of the penis from the vagina just before the man's orgasm, cannot be considered a very effective method of contraception (birth control) and is not recommended.
Sexual intercourse is also an effective means of transmitting sexually transmitted diseases. Health care professionals suggest that condoms should always be used, but they should by no means be considered an absolute safeguard. The best suggestion is to avoid sexual intercourse with anyone known to have a sexually transmissable disease, and indeed anyone whose disease-negative status you aren't certain of.