What are public interest pressure groups?

What are public interest pressure groups? Describe their functioning.

Public Interest Group :
Public interest group or public interest pressure group are those groups which seek to promote the interests of the large section of society not for its member only. For example, a group fighting against bonded labour or child labour fights.
Functioning of Public Interest Group:
Sometime the members of a public interest group may undertake activity that benefits them as well as others.
For example, BAMCFF is an organisation largely . made up of government employees who campaign against caste discrimination. It fights a for the problem of its members who are victimised by caste discrimination. But its main , concern is with social justice and social equality for the entire society.