What is the meant by snowball sampling cluster sampling?

Snowball sampling is often used when interviewing. Instead randomly asking people about a particular topic, you would interview initially a person thought to be knowledgable about a subject and then ask them to identify additional people who could serve as usefull interviewees. You then interview those people and ask them to suggest even more people. Thus, your pool of interviewees increases over time, something akin to making a big snowball where it slowly groes as you add more snow.