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.