ehhh there are a few ways to tackle the situation...
F1 cars are the quintessential race cars. Open-wheeled, they are designed to create maximum performance mostly through the synthesis of both mechanical engineering and aerodynamic engineering because the tracks are so twisty.
You see the highest degree of cutting edge technology in F1: all carbon-fibre monocoques, magnesium alloy transmissions, wind tunnel testing, F1 designed gearboxes...basically all this technology is supposed to eventually trickle down into the various company's road cars (i.e. Ferrari F2005-->Enzo-->FXX).
The league is regarded to contain the best drivers in the world (Schumy, Kimi, Alonso, the late drivers such as Senna, Prost, Mansell etc...) to compliment the machines which are the most advanced and cost the most (avg. F1 car costs 14 million just for production compared to a NASCAR which costs about 150k, which doesn't count the F1 car's hundreds of millions for testing creating and maintaining). Basically F1 is the pinacle of racing. However, a lot of people are turned off from F1 (especially in the US) because of the "politics" involved. For example, last year, regulation changes to the rules on tyres effectively sabotaged Ferrari's stranglehold on the sport.
Also there are no American drivers, no American race teams (albeit Red Bull is thinking about acquiring Scott Speed; again, political). F1 races are often decided in qualifying (i.e. Monaco), as the driver with the best car/most talent rarely makes mistakes to allow others to pass. Passes are infrequent, except maybe at the starting grid where crazy drivers can manufacture a lead (Sato!!) so many non-fans get bored watching the grid at the top stay the same. However, when there are passes, they are often the most spectacular in all of racing (e.g. Kimi's pass on Fisichella at Suzuka! WOOOOOAH!!)
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LeMans and CART/ Formula3000 etc etc are all about one step below F1. Less money= less technology= (arguably) less talented drivers (F1 is so political, albeit all the drivers are great...).
LeMans cars are designed for high speeds. Often time their topseeds are higher than the F1 cars because the courses are a bit less reliant on downforce. They are of completely different build design (non-open wheel) and have far less design restrictions (arguably) than F1. LeMans cars are about endurance as their races are often excruciatingly long. The drivers most have consistency and endurance. LeMans races often race alongside GT and lower Cup race settings (hence why you see these flat LeMans cars racing alongside 996s, Mustangs, F360 Challenges etc etc). They do this to spice up the racing (creates more passes between classes) and also to keep the cost down for the event.
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Champ Car/F3000 is something like an F1 derivative (open wheel) but with far less money and a more American fanbase. Less money= worse drivers, worse cars, but still pretty damn enjoyable stuff to watch if you know a lick about racing. These guys are often a bit more crazy than the average F1 driver who has so much riding on him that he needs to play it safe...Lots of good drivers come up through here (JP Montoya, Sato, Albers etc etc...)
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Indy Racing League is similar to F1 but the aerodynamics of those cars are pretty much bollocks because their courses are primarily ovals (albeit they have some twisty courses thrown in there). Indy cars can achieve speeds up to 220mph which is higher than most top speeds on F1 cars because they dont really have to manufacture any downforce.
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World Rally Championships is a racing league that involves offroading 4WD vehicles (WRXs, Evos, Citroens etc)... I find it extremely pleasureable to watch due to the HIGH risk involved in the treacherous gravel/dirt/snow roads. The driver is unaware of the course ahead and has a cockpit navigator who has driver the course, who has notes and reads him advice for upcomming turns (e.g. "Hard right, 2nd gear comming up). Very dangerous stuff, great to watch. Non-race format due to the windy roads, its a time-trial format.
Hope this helps...ask specific questions cuz I can't empty my whole brain out on the page