Cluster rings have been worn at least as far back as the Georgian era beginning 1747 and have continued to be popular to this day.
There are many variations on a cluster. While there is a classic cluster which is simply round or oval in shape featuring a prong set round cut stone in a halo around a central gemstone. There are many varieties of cluster-like settings that have either evolved from or are simply cluster-like.
These can then be broken down further for example a target setting which has two rows of gems instead of one or a halo setting which is perfectly round and has channel set gems as the cluster. But these are all interchangeable. Sometimes a halo is called a cluster and visa-versa. As explained above we usually stick to those definitions but there are no set rules.