In some eastern cultures there is the harem, where one man has more than one woman for sex, status and variety. One of the harem women is normally in charge of the others. In western cultures the similar arrangement is called polygamy (and is usually outlawed when the man marries more than once without divorce). Often the women and their children live separately with the man sharing his time between the various homes or the man has left a marriage and remarried without divorcing first.



Then there is 'having affairs' when one or both partners have a separate relationship usually secret and usually 'part time' and temporary.

Different from any of these is 'swinging' where (usually both partners) are involved in sexual 'fun' with other people, often together, without emotional 'love' ties with the people outside their relationship.

More rarely a three way relationship can develop where all three people live together (menage a trois) in a sexual and emotional relationship (together with children if any).

Polyamory is similar to three people living together but goes further. The term can be applied to any sort of open relationships but it often involves a sort of commune of two or more 'families' where there are emotional ties and sexual relationships between various people in the group. The ties and sexual activities are not necessarily 'even' throughout the group but, while the group is successful, all the (adult) participants agree to the various intra group arrangements.

The advantages of polyamory include the chance for people to vary and satisfy their sex lives and their intimate social lives with a wider variety of people within the group without the difficulties with cheating on a partner or the issue of jealousy.

For example there might be two couples that join together with one extra woman as was the case described in the New Scientist (8 July 2006). In this case one couple had had relationships with other people, but instead of their relationship splitting, the other people joined up with them and a third woman joined to create a complex group of two men, three women and one child (with another on the way).

These polyamorous arrangements can obviously become very complex, both physically and emotionally. Infidelity in monogamous relationships is estimated to be between 60% and 70% and is often the cause of the breakdown of the original relationship. You might have thought that polyamorous relationships would be less stable than conventional ones and even more prone to breakdown, but apparently the limited amount of evidence that there is, suggests that 'poly' couples stay together just as long as monogamous ones.

However with polyamorous arrangements often being very flexible I am not sure how they measure that!

You will find an excellent definition of polyamory at wikipedia.org.