So basically...go play the game and don't try figuring this out? =p
Not really, you can figure it out, but because I can't check my garage and see the cars, names and descriptions it's difficult to be precise about all race cars in all series that are in the game. GT5 covers the past 20 years and most of the major race series from around the world, during that time there has been many changes to class regulations, cars and what the series / classes have been called.
Generally, it's not too bad if you just want a certain type of car i.e. Le Mans 'top' spec, DTM or JGTC, but if you're going to catergorize all cars into classes and look at the differences, it might take a little time.
Most of the top spec Le Mans cars have certain similarities, for example weight, most of the cars would be between 850-950kgs and have 700-950bhp.
The lower spec cars would be heavier, around 1100kgs and have less power, but like I say, there are so many variations, being general isn't easy, where's being specific is alot easier i.e. possible Group C cars I can think of are the Nissans R89 / R92's, Toyota Minolta, Jaguar XJR '88, Sauber C9, Peugeot 905.
Group C was abolished sometime in the early 90's and replaced with (I think, but not sure) LMP class, like the Audi R8 '05, Pescarolo (there's two, can't remember which one

). Then the Bentley came along with similiar specs but actually raced in a different class. Also, the name for the class in which these cars raced in varied between Le Mans, America and Japan, but the cars were very similiar specifications.
But, like I say, I can't check my garage at the moment, so I'm guessing a bit, sorry.
EDIT:
Just finished my B spec endurance, had a quick look in the garage and the description. An example:
Nomad Diablo GT1 - this started out as a JGTC car, but was then changed to a GT1 specification car. Check the description of each car, this will make it alot easier to understand the car and what class it was in, plus the description will be alot more accurate than me!