Enhancing equipment, oh the joys and troubles this caused me when I first started playing. The problems I faced were numerous, I didn't know what my secondary stat priorities were, I didn't know what gems to look for, and I didn't know which socket bonuses to meet. Eventually, with the help of Elitist Jerks, and the askmrrobot website, I figured these things out, and I now keep a few of the gems I always need for new gear in my bags.
Knowing which gems to use is imperative for me as a raider, when I am trying to min-max. (Wiping with the boss at 20k health has happened once, and we've even had some .5% wipes.) There are two ways to gem correctly, however one is better, and probably cheaper, but requires more knowledge. The easiest option is to just gem your primary stat. I should mention that this is for healers and DPS only, tanks vary by class, gear, and what you are raiding. If gemming straight red gems, you'll be using either brilliant, delicate, or bold inferno rubies. Make sure that your meta gem is active, however. You might have to use a combo gem or two, to activate it. This method is more costly, as on most servers red gems are the most expensive. You lose out on socket bonuses, but in some cases this is better then losing your primary stat, depending on the bonus, and comes with the ease of always knowing what to throw in a socket.
The second option requires you to know stat weights. For me, as a fury warrior, my stat priority goes like this, 26 expertise->8% hit-> crit->hit (up to 26%)->mastery->haste. The stat weight of Strength:Critical Strike is .66 roughly, or 3 points of critical strike is worth about 2 points of strength. This means if the socket bonus is +10 strength, with a yellow socket, I will use an orange gem, in my case inscribed ember topaz, which is +20 strength and +20 critical strike. Thus, the bonus from that socket is +30 strength and +20 critical strike, which is slightly better then the straight +40 strength of a bold inferno ruby. Hit is worth slightly less then half a point of strength, however because I need more hit when I go arms, etched demonseye is a quarter the price, and I am OCD about meeting +strength bonuses, I will use this purple gem in blue sockets. As well, I'd like 26% hit soon, for my white hits to always connect, granting more constant rage, and evening out my rotation. With changes, these stat priorities and weight change as well, buffing white hits for fury warriors made hit worth more after 4.1, for instance.
Knowing which gems to use for your class will help you play better. If you are a jewelcrafter, it should also help, as you can learn which gems will be in the most demand by raiders. I'll always be using bold inferno, etched demonseye, inscribed ember topaz, and reverberating shadowspirit diamond. When my Jewelcrafter hits level 75, the first patterns I will buy will be all three +40 agility/strength/intellect infernos, and then the correct demoneyes and ember topaz cuts for various classes, followed by other gems needed. Knowledge is power, which can be utilized to make money.
Now, to explain something people consistently demonstrate they don't understand: Reforge for those stats, don't gem! I cringe when I see someone with a hit gem. You never should do that. You can reforge out of a lesser stat, such as haste in my case, to hit the soft expertise cap. By doing this I turn a stat worth less into a stat worth more, and I can gem my primary stat, strength. If I gem hit I lose that strength. You can't reforge into your primary stat! You are giving up that stat for a lesser one. It would be better to be at 7.97% hit as a hunter then to be 8% hit and lose out on that agility. REFORGE! Websites such as askmrrobot are AMAZING at helping you optimize, so as not to be over hit cap, and to utilize your reforging to give the best boost.
Learning to gem will help you not only do better, but will show you know your class better as well. You can also profit off this knowledge of your class, or other classes as a jewelcrafter. Learn2gem, and profit off that knowledge!