Besides legalities, what are the similarities and differences between "open marriage" and polygamy?

I understand “open marriage” to be a mutual agreement between spouses to explore love and sexual relationships outside of marriage, and that it is not the same as polygamy because additional spouses are not added to the marriage. I’m sure there are other definitions of “open marriage,” and I’d like to hear them, too.

I understand polygamy generally means that the husband has more than one wife, and sometimes the entire cluster of families co-exist. Again, I’d like to hear other definitions.

Essentially, though, in either case, the husband and wife (or wife and wife, or husband and husband) are not exclusive. They are sharing their marital intimacy with others. Is this correct?

According to your understanding of “open marriage” and polygamy, what would you say are the similarities and differences, in terms of:

1. Equality of individual spouses,
2. Positive and negative effects on the marriage, and
3. Positive and negative effects on the individuals involved?