Some populations that we are interested in studying can be hard-to-reach and/or hidden. These include populations such as drug addicts, homeless people, AIDS/HIV patients, prostitutes etc. The populations can be hard-to-reach and/or hidden due to social stigma, illicit or illegal behaviours, or other traits that makes them atypical and/or socially marginalized. Snowball sampling is a type of non-probability sampling technique i.e., the technique is based on the judgement of the researcher, and can be used to gain access to such populations.

To create a snowball sample, there are two steps:
1. First, we need to try and find one or more units from the population we are studying. Ex: If we want to study students that take drugs, the aim is to start with whatever small number students, who are willing to participate, even if it’s one or two units.
2. Using these units to find further units and so on until the sample size is met. Ex: The researcher asks the students who participated in the research to help identify other students that may be willing to take part. In this respect, the initial students help to identify additional units that will make up our sample.

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Source:
http://dissertation.laerd.com/snowball-sampling.php