First Aid is a secondary profession skill, that can be learnt by all players in addition to their two primary crafting trades.

First Aid lets you create bandages that you can use to heal yourself (or other characters). They are very useful when you are out on your own, speeding up levelling or farming by decreasing downtime. Furthermore, it is also viewed as essential in raiding guilds as there will be times where you will be expected to keep yourself alive rather than relying on your healers (unless you are a tank of course ^.~).

The profession is very easy and cheap to skill up - the only annoying part is that you have to travel around a bit to visit your trainers.