Unlike Polyamory and polyamory dating where you are committed and in love with multiple people at the same time, an open relationship allows a couple, married or otherwise to date, see, and be sexually involved with other people outside the core relationship(s) on a generally more casual basis.
For some people, the responsibility of polyamory or being in love and responsible to several people is more than they can handle, so for these individuals, an open relationship is what they need.
Many people have different types of people as their outside partners - i.e., people that are best friends, neighbors, people they meet on the internet, or even strangers for a fling.
In an open relationship the possibility for jealousy increases when the core partners do not meet the outside partners.
Open relationships are an alternative to cheating, and are many times used because of conflicting schedules, distance, time availability or just basic curiosity or desire of the core partners.
Open relationships can save a stale marriage or infuse new and different energy and vitality into an existing relationship.
Open relationships and open relationship dating, are becoming more and more common all over the world. It has become so commonplace that even celebrities are including open relationship clauses in their marital contracts.