Ok I will try and give you a short run down of the jig classes
Swim jig, a jig meant for swimming ;D forward line tie, eye of the hook inline with the line tie, usually a lighter wire hook that helps the jig stay upright while deflecting off of cover (most companies make a heavy duty version also for those who don't trust the light wire hook) usually has a lighter swept back weed guard also, to help in hook sets since you get a lot of fish slashing at the bait. also usually has a thinned out skirt to aid in action and profile Uses: swimming anywhere from heavy cover to open water, usually a 4-5" grub as a trailer but can also use swimming type chunks (paca or rage) or paddle tails like a Keitech. Think of these jigs as a spinnerbait without the blades, you fish them in the same type of situations except they go through cover better and are alot more subtle.
Grass: Looks similar to most swim jigs but has a heavy wire hook, heavy weed guard. has a forward line tie with the weed guard swept back, has a full skirt and usually rattles. there is lots of different variations and some work better than others but if you are flipping or pitching grass or pads or any other "junk" look for a jig that has a forward line tie, swept back weed guard, and a heavy hook.
Football: Shaped like a football, it craws over rock and gravel with less of a chance of hanging up. There is a ton of variation to the football jig, some have heavy hooks, some have light hooks, you have to try different models to figure out which works best in what situations. But remember when you are fishing deep rock you usually have alot of line out and hooksets become easier with light wire or high end hooks!
Finesse: Usually a round head jig with a flat eye hook, the hook is usually a regular or thin wire to aid in hook sets with the lighter gear these smaller jigs are usually fished on. Used in "finesse" situations. Either pressured fish, high pressure situations, or a change of pace to the normal full skirted jig. These jigs are versatile and a huge part of my jig arsenal! They have a completely different profile in the water, you can flip them into lighter cover, drag them, skip them, basically anything you can think of!
All Purpose jigs: Most jigs you see fall into this category, my alien head or arky head jigs are a good all purpose jig, they are especially adept around wood or or gravel, come through vegetation nice (not slop just normal vegetation).
When you are looking at jigs it can be overwhelming, but probably 75% of normal jig fishing can be covered by a good all purpose jig. If I didn't make my own jigs I would probably have mostly all purpose jigs with some fb heads, finesse heads, grass heads, and swim jigs thrown in. But if you are just getting started into jig fishing I would recommend some cheap all purpose jigs to get started with and then learn from there. Once you get better at jig fishing you can expand your arsenal a little and figure out what works for you and what doesn't