The Guild Wars site has put up a helpful FAQ for the Skill Unlock Packs they are offering now. But firstly, what IS a Skill Unlock Pack?
Whether you're a PvE or a PvP gamer, you'll find the new Skill and Item Unlock Packs hard to resist. The new Skill Unlock Packs provide access to basic and elite skills from all Guild Wars® games (the original campaign, Factions, Nightfall, and Eye of the North) for your use in PvP combat, and give your Heroes an advantage over your foes in PvE. The new PvP Item Unlock Pack provides hundreds of item upgrades to ensure that you go into combat with exactly the gear you want.
If you've got questions, they've got answers right here.