Purposive sampling starts with a purpose in mind and the sample is thus selected to include people of interest and exclude those who do not suit the purpose.
This method is popular with newspapers and magazines which want to make a particular point. This is also true for marketing researchers who are seeking support for their product. They typically start with people in the street, first approaching only 'likely suspects' and then starting with questions that reject people who do not suit.
Purposive sampling is non-probability and hence can be subject to bias and error.
The main goal of purposive sampling is to focus on particular characteristics of a population that are of interest, which will best enable you to answer your research questions. The sample being studied is not representative of the population, but for researchers pursuing qualitative or mixed methods research designs, this is not considered to be a weakness. Rather, it is a choice, the purpose of which varies depending on the type of purposing sampling technique that is used