APPGs are informal cross-party groups that have no official status within Parliament. They are run by and for Members of the Commons and Lords, though many choose to involve individuals and organisations from outside Parliament in their administration and activities.
Groups which register as APPGs have to conform to rules about cross-party membership and transparency about their activities and funding. The Register of All-Party Parliamentary Groups is an authoritative list of such groups. A new edition of the register is usually published every 6 weeks approximately.