Public interest groups differ from other types of interest groups in that

they claim to serve the common good, not just their own particular interests

Explanation:

the majority of interest groups actually controlled by groups or individuals that will benefited from a type of legislation. Which is why they help fund the campaign of government officials in order to make them indebted to them.

Public interest groups make the claim that even though they acknowledge they are promoting a select view of government officials, they do it to pass the legislations that will benefit the society as a whole (such as human rights and environmental protection)