What is the difference between stratified sampling and cluster sampling?

Cluster sampling is sampling in which groups not individuals are randomly selected All the members of selected groups have similar characteristics this method is used when the population is very large and heterogeneous and widely scattered stratified sampling is the process of selecting a sample in such a way that identified sub groups int the population were represented in the sample in the same proportion that the exist in the population when the sample size is small and homogeneous this technique is used