This is one of the most interesting non-probabilistic techniques. You first select, at random, members for the sample. Suppose you selected 3 members. Now, these three will suggest more names for the study, and this creates a chain effect.
Snowball sampling is useful in cases where it is difficult to locate people, or they do not wish to be identified.
For instance, in medical research where you are studying a rare disease, you might find that snowball sampling is the only way you can get to the desired sample size.