People often ask what the difference is between strata and clusters in sampling. Here’s my current soundbite version
Strata are a partition of the population where we hope different strata are different. We sample from all the strata in the population, so the generalisation from the sample to the population is within strata and is more accurate because of stratified sampling.
Clusters are a partition of the population where we fear different clusters are different. We are forced to sample from just a few clusters in the population, because it’s cheaper that way. The generalisation from the sample to the population is between clusters and is less accurate because of cluster sampling.