The first leter just tells you the insert shape, which lets you know what the angle. There are all types. T is 60, W is 80, as is C. The nice thing about the W over the C is that you get 50% more edges on your inserts (3/6 vs 2/4). In general the wider the angle, the more abuse the insert will take. Another thing to consider is the tip radius. The larger the radius, the tougher the insert will be, but there is a minimum depth of cut you have to use (IIRC it is >= to the tip radius). If you don't have a lot of power/rigidity, probably best off with an insert that has a small radius. Of course, none of this has anything to do with the holder you get... which may seem confusing, and I don't know if I can do it justice, but I'll give it a go.
For cutting tools, there are a couple angles that people talk about (f you have tried grinding your own HSS tools, some of it may be familiar), relief, and rake (sometimes broken into side and back rake). Relief is the angle of the cutter below the cutting edge on a turning tool. This angle keeps the freshly cut material from rubbing on the cutter. Rake is the angle the top edge of the cutter makes with the horizon, and it makes the cutting edge more or less "keen".
In carbide inserts, there are three ways that you can get the right combination of these two angles; they can be ground into the insert itself, they can be machined into the holder, or it can be a combination of both. The most common way to get relief is to get it from the holder. These are "neutral" (second letter of the insert code is an N) inserts. If you place them face down on a table, the sides will be perpendicular to the table. If you fed them straight into the workpiece, there would be a lot of rubbing, and a lot of heat; bad times. To use these inserts the relief is built into the holder, specifically they hold the insert at an angle from horizontal (usually 7 degrees). The nice thing about neutral inserts is that you tend to be able to use both sides of the insert, so you get twice the number of edges out of an insert as a "positive" insert, for about the same price per insert.
So now we need rake. I'll focus on neutral holders again (because that's what I recommend). For really tough stuff, you can get flat inserts; 90 degree cutting edges. They are really tough, and good for hard things, but for softer material, they tend to just mash things around rather than cutting. The trick with rake and neutral inserts is in the chipbreaker. Chipbreakers are a trough ground into the top edge of the insert. Their main purpose is to break chips (hence the name), but they also effect edge geometry (they make the edge keener). This is really important for softer materials, in fact there are a lot of specialize aluminum inserts with really deep chipbreakers and very sharp ground edges.
So, the only things you need to figure out when starting out are the first two letters of the inserts you want to use, and the size. I recommend neutral inserts, you will have the most choices. For the shape, I recommend T, W, or C, as they are the most common, and you will have the best selection, lowest prices.
At that point you have your holders, and you have to find some inserts for your material. Take a look through the catalogs and find something recommended for stainless, and remember that the holder will take a wide variety of inserts. You can buy singles of a couple different types to try to see what works best for you, though it sounds like you have some random inserts, so give those a try, figure out what you like, and order more like those.
If you have stuff on hand you are going to have a hard time identifying the carbide grade unless they are marked. I wouldn't worry about it too much. They will cut, they just won't be optimal; when you run through those (assuming you don't have an unlimited supply of free inserts), you can worry about getting the "right" grade when you need to purchase more.
