As tIANci pointed out, most categories are based on what the plane is used for. These include: Combat, Pattern, Pylon, SA (aka IMAC) (Scale Acrobatics), trainer, sport, 3D, fun fly, etc.

Some categories are based on size, such as the 1/2A and 1/8 as you mentioned. You also see "40" for planes designed around .40 class engines, for example. "Giant scale" usually means 1/4scale or 80" wingspan, "gasser" for a plane powered by gasoline rather than "glow" fuel or "electric".

You can combine categories. So you could have a 1/2A trainer, or a giant scale 3D plane.

There are also planes that fit more than one category, and planes that really don't fit any of them very well (Though "sport" is pretty much the "everything else" cagetory).

With experience, you can start to pick out what a plane is and how it will fly. It's a lot more than wing location, but you can tell a lot when you know what to look for. (Some, but not all, of the factors that make a Citabria NOT a trainer include the short tail lengh (tail moment), small tail size, airfoil, landing gear configuration, and wingloading (weight / wing area). Of those, the only one you can't tell by looking at it is the weight, and you can get that from the specs when shopping.

Also, note that most (but not all) of the useage categories up top are based on specific contest rules and regulations, or some variation of those. Those of us who fly in competitions obviously fly planes designed for that competition. We might also fly the same planes when just out goofing off, ie for "sport".

However, the big thing is to just not worry too much about it. It's less important than you might think. The most important thing is to start with a plane that is good to start with and work up from there. Once you get the basics down, you'll pick up enough info along the way to.