Interpersonal relationship

The phrase personal relationship characterises some sort of connection between two or more people.

It includes:

Marriage, civil union
A loving relationship or romantic relationship with or without living together; the other person is called lover, boyfriend or girlfriend (not to be confused with "just" a male or female friend) or significant other; there is usually sexual behavior, but often in the beginning only limited forms of that. If the partners live together the relationship may be similar to marriage, and the other person may be called husband or wife, and regarded as such in common law. The term sexual relationship may be used, even if it involves more than sexual behavior, if the latter is perceived as the most important distinction from friendship. "Mistress" is a somewhat old fashioned term for a female lover of a man who is married to another woman, or of an unmarried man. She may even be an official mistress (in French maitresse en titre); an example is Madame de Pompadour.
A sexual relationship in the more literal sense: one that mainly involves sex.
Friendship
Acquaintanceship
Family tie, being relatives, kinship, biological relationship
Brotherhood and Sisterhood

A friend of a friend of someone may well be a friend of him or her, there is some transitivity. However, if two people have a sexual relationship with the same person, they may well be competitors rather than friends. Accordingly, sexual behavior with the sexual partner of a friend may well damage the friendship.

In a marriage or loving/sexual relationship there is often, but not always, an implicit or explicit agreement that the partners will not have sex with someone else. The extent to which physical intimacy with other people is accepted may vary. For example, a man may accept more physical intimacy between his wife and a female friend of her than if it is a male friend. (See also jealousy.)

The rise of individualism and of psychology may have led to the explosion of concern about one's personal relationships (or, in popular parlance, simply: "relationships"). Modern popular culture expects relationships to exist and to become laden with depth and meaningfulness. Pair-bonded sexual relationships receive particular attention in this context, but sociology recognises many other inter-personal links of greater or less duration and/or significance.