Alchemy gives players the ability to craft potions, poisons, dyes, and tinctures.
Poisons are primarily used by the assassin classes (Infiltrator, Shadowblade, and Nightshade) for dealing extra damage and negative effects on their targets.
Potions are basically a bottled spell, which will cast itself upon the user when consumed.
Dyes can be used on armors, shields, and cloaks to change their color.
Tinctures are used to place special spells on other crafted items, in the form of charges, procs, and reactive procs.