Engineering is one of WoW's more interesting crafting professions - it requires materials produced with another profession, usually Mining. Engineers turn various reagents (most parts and metals) into fun and useful gadgets, vanity items (like mounts and pets), and even some epic helms/goggles, ranged weapons and scopes for them.
Most of the items crafted by engineers have little real combat application. They focus on fun and/or useful gadgets like Goblin Barbecue or Loot-A-Rang, but you will see many engineers proudly wearing their Deadly Bio-Optic Killshades, socketed with two Engineering-only Cogwheels, shooting their enemies with Kickback 5000 - enchanted with Gnomish X-Ray Scope, of course. Oh, and throwing Big Daddys. And then riding away with their Mechano-Hog. When toss personal repair bots, teleportation devices, pets, and various gadgets (e.g. World Enlarger), Engineering becomes one of the most fun professions in the game.
Profession Perk
Engineers get quite a few perks. Special trinkets and fun items notwithstanding, the real profession perks are: epic engineering goggles that make for a very nice helm, Cogwheel helm gems as good as the Jewelcrafters' own gems, and fun gadgets to use as enchants for their gloves and belt.