It's all pay as you go, no TV is free unless you have a antenna on the house.
I do.
and most programs are not supporting an Apple IMac.
Not sure what this means. AppleTV has nothing to do with an iMac either. Or MacBooks, or iPad. (Although It does work nicely with those too, if you have them.)
You point the remote at it and watch stuff. There's the following items in the on screen menu...
Movies - This is anything they sell or rent on the iTunes store.
TV Shows - Same thing.
Music - links to your iTunes music connection stored in iCloud, if you have one. If you pay the annual fee, also stores your non-iTunes MP3 formatted stuff so it's playable anywhere on any Apple device.
Internet - This is where we spend most of our time. Includes built in support for Netflix, iTunes Live (Apple sometimes has free live concerts), NBA, NHL, MLB.TV, WSJ Live (Wall Street Journal), YouTube, Vimeo, Podcasts (will only link to podcasts syndicated at the iTunes Store), Photo Stream (Apple's thing that shows all your Apple device photos without having to copy them between devices), MobileMe (view photo galleries which will be going away with iCloud but still works now), Flickr (great way to view your digital photos and friends photos), and finally Radio (which is hundreds of various Internet audio streams of all sorts.)
Then there's the Computers tab, which is what you're thinking. You can play most media that's in any machine running iTunes, including PCs I believe. The AirPlay feature enables this the other direction.
I use AirPlay all the time. "Hey honey, check this video out I found!", press play on the iPad or iPhone and switch the output to "AppleTV", and it pops up on the TV and starts playing. Very cool. We leave the AppleTV on as a giant photo gallery screen saver of all our favorite photos whenever we're just sitting here in the living room reading or working on the laptops. Makes tossing something up on the screen to share a piece of cake.
When I'm out and about and someone sends me a funny YouTube video, I just add it to my "Favorites" list at YouTube and when I get home, watch it on the big screen.
In all, for $99 plus a cheapie Netflix subscription, it does a ton of stuff. No other Apple devices required. But they're all integrated nicely if you do have other's. Or a PC running iTunes.