Meggs,
I still haven't seen a simple Janet and John answer like the sort that I sometimes need, so;
Bleeds are the basically the mechanisms used to take very hot air from the engine for a variety of uses, including pressurisation.
Packs are the pieces of equipment that bleed air must pass through to bring it to a sensible temperature before it can be used for cabin/hold pressurisation.
In essence, bleeds are stage 1 and packs are stage 2 of the pressurisation process.