As a spellcrafter, you imbue gemstones with magical properties - strength, resists, etc. Then you apply the gems to high quality player-crafted armor and weaponry. The skill gains come from making the gems - for the sake of our programmers, the game will assume that if you have the skill to make the gem, you have the skill to apply them. Of course, if you go beyond certain parameters (you can only add so many gem properties to a piece safely) and try to over enchant an item, there is a chance the item you're trying to decorate will blow up in your face and injure, maybe even kill you.
Only casters can be spellcrafters - pure melee and hybrids won't be able to even try.