If the world of Guild Wars exists in an uneasy balance between technology and magic, then the engineer can't be faulted for letting it know where her allegiance lies. Utilizing grenades, turrets, and many other sorts of whiz-bang technological terrors, the engineer are the MacGyvers of Guild Wars 2, capable of bringing to bear an immense amount of firepower on an enemy at the drop of a hat. Engineers only have a few weapons available to use, but that's due to the fact that they can "equip" extra skills with device and weapon kits, effectively giving them completely exclusive skill bars that no other class has access to. As a result of this, engineers are not capable of using the weapon-switching button.
Said kits include three weapons (a flamethrower, an elixir gun, and a tool kit) and three device kits (a bomb kit, a grenade kit, and a med kit). When equipped, your normal weapon skills will be replaced by those of the kit, so be sure to learn all of them and switch as appropriate, given the situation.
In addition to the kits, engineers are renowned for their knowledge of turrets. They can place many different kinds of turrets, enabling them to defend an area while still forging ahead with their own combat acumen. There are rocket turrets, flame turrets, rifle turrets, and even healing turrets that can be used. Know them and love them! All of the turrets can be self-destructed to deal damage to nearby enemies.
Another addition to the Engineer's bag of tricks is the tool belt. The tool belt effectively adds a separate secondary skill above the engineer's healing skill and utility skills, giving you an even wider array of options than you would have normally. For instance, if you use Elixir B to gain boons for yourself, your tool belt will add a second option, allowing you to Toss Elixir B to grant a random boon to nearby allies. There are dozens of these modified skills to play around with, so check them out and see how your favorite utility skills are affected. Even the racial utility skills get tool belt modifications!
The engineer can take advantage of the Tool Belt Recharge Rate trait line to allow his tool belt-modified skills to recharge faster.