Class skill: scout
Repair is similar to the computer use skill in that the difficulty of successfully repairing a damaged machine or droid is not based on the use of a DC check. Rather, the repair skill merely reduces the number of parts required to repair a broken droid, allowing you to save one part for every four ranks you possess. It's worth pointing out that, as you progress further into the game, most of the broken droids you encounter will require multiple individual repairs before they're restored to working order, and since you save parts from each repair, you can sometimes save four or five parts per droid for every four levels you possess in repair. Or you can just skip repairing the droid, and make up the couple hundred experience you miss out on by killing one extra enemy somewhere along the line. Repairing droids can be amusing every once in a while, but the occasions where they accomplish what your party alone could not are, well, non-existent, at least in this game.
Repair also governs the amount of vitality that your droid NPCs gain when they use repair kits on themselves to recover from damage.
Class skill: scoundrel
What would a BioWare RPG be without lockpicking? Security is the euphemism used for that eternally useful skill in KOTOR; characters with at least one point in security can attempt to disable the many locks that you'll encounter throughout the game, with higher skill required for more difficult locks. This skill doesn't necessarily require equipment, although you will encounter lockpicks (excuse me, "security spikes") throughout the game, which can add a positive modifier to a lockpicking attempt. All of these spikes are one-use-only, unfortunately, and are removed from your inventory after you add their modifier to an attempt to breach security (whether the attempt was successful or not).
Of the three tech-oriented skills, security will generally be the most desirable. Again, the game has been carefully crafted to ensure that you never have to have a character with a high security rank in your party, through the use of keys and passcards and such, but most areas will have numerous locked stashes of items that you will miss out on without at least one character with sufficient security to open them. On the other hand, you will be able to make do without security if the locked item is susceptible to bashing; to see if this is possible, use the d-pad to cycle up or down from the "Open" command on your action menu. If the attack icon appears when you have a locked item in your reticule, you can simply hack it open instead of picking the lock. This works on a surprisingly high number of locked doors and containers, but keep in mind that many of these bashable items have a very high damage resistance, meaning that it might take you a very long time to open them without the security skill.
Class skill: soldier, scout
The best armor in the galaxy won't prevent your character from taking damage every once in a while. You have a finite amount of vitality, and this precious commodity does not recover by itself over time; you'll either need to find a doctor to repair your wounds, or consume a medpac to gain back your health. The treat injury skill simply adds itself to the amount of vitality you would normally gain from a medpac; this amount can be further increased by using advanced medpacs or life support packs.
Players who intend to become Light Jedis would do well to keep in mind that the cure and heal Force powers essentially replicate the effects of medpacs, and have the bonus of being reusable, since your Force gauge will gradually replenish itself over time. If you plan to focus on using Force powers, you may want to tough out the early sections of the game without over-investing in treat injury if the skill will become less useful when you gain the ability to heal yourself with the Force. On the other hand, soldiers and those who wish to become Dark Jedi will probably want to pump their treat injury skill quite a bit, since it will heal them more than their Force powers will be able to.