Doors are annoying things that get in your way most of the time. It's typically left open or broken. Engineering allows you to fix doors using a Portable Toolkit. Fixing doors costs 5 AP and gives you +1 Morality (MO) and +5 Experience Points, breaking doors give you -1 MO. You can repeatedly break/repair doors and still gain experience off it from the repairs. Repairing a broken door brings it to 10HP, each subsequent repair costs 2 AP and adds another 10HP up to the maximum of 30HP, awarding 2 Experience Points in the process but leaving the character's Morality unchanged.
Doors have 30 HP, 0 Soak and a -25% defense value. They can be targeted with Spells directly and via Martial Spellcraft as normal (just hit it with your weapon). However, doors don't give the bonus damage from being Hidden nor from a Critical Hit.
Doors can be locked from the inside. While locked, they cannot be opened from outside. The lock can be picked (especially if you have the Lockpicking skill) or the door can be forced open by smashing it with a weapon. Some Tier 3 classes can bypass locked doors entirely. Locked doors (like Glyphs of Protection) never prevent people from exiting a building - in the case of a locked door the character is assumed to have simply exited via a window instead.