You can only combine Relics and charms of the same kind, and you can only add one kind to a piece of equipment. You can choose to combine the relics/charms as you find them, or you can use them on a piece of equipment right away. But after that you can only add the same kind again. They are bundled in 3's or 5's and upon completion, they give an extra bonus (expect Boar Hides at least). I got a few from quests too, but most I got from mobs. There are relics for almost everything, so its just a question about preferences and good 'ol luck
(and FYI, I just upgraded my comp a week ago but haven't had anything to play since, so was in serious need for some g33k-time
)