This is one of those rare occasions where I think wikipedia-ing the information will get you as much as asking here. There is relatively little to say that isn't provided there (except that Geoffrey's death forced Henry II to abandon his campaigning in Wales to rush to secure Nantes). They were younger brothers who were accorded lands (the county of Anjou and the viscountcy of Dieppe respectively). They practiced lordship over those territories, bickered with their neighbours and eldest brother, then died at rather important points in Angevin history (the first Welsh campaign and the beginning of the 'Beckett Affair').

For a wider context of Henry II's reign see:

Carpenter, D.A., The Struggle for Mastery: The Penguin History of Britain, 1066-1284, (London, 2004).