Dumke, to help you out a little more I'll need some more information. I get that you don't want a road bike (usually lighter bikes with skinny little tires), but do you think that you'll get pretty gnarly off road, or will you be, mostly, on gravel paths when you're "off road"? I ask because you could get a beach cruiser type bike. They are usually very comfortable with big cushy seats and wider tires. You also usually sit more upright on them, but they usually only have one gear (some have three and others seven). Something like this: http://www.harborcountrybike.com/Ele...e_p/168260.htm Electra makes a bunch of cool bikes actually. Check out their website: http://www.electrabike.com/home.php Most of the cruisers won't allow you to stand up and pedal due to their laid back riding position.
The next step up would be a mountain bike. They have wider knobby tires (usually), but you're in a more hunched over position when you ride them. You can usually stand up to pedal, and depending on the frame and bike, you can get into some mild to serious off roading. Once you're in the mountain bike territory you can get anything from completely rigid to fully suspended. A rigid bike is just what it sounds like; no suspension. Those will be the least expensive, but most "uncomfortable". The next step up would be front suspension. This means that the rear tire is mounted to the frame and the front tire is mounted to a suspension fork. This will allow the front wheel to absorb some shock as you hit bumps and curbs. The final step up is a fully suspended bike with front and rear shocks. By far the most comfortable, but probably the most expensive of the bunch. You can still get, fairly, inexpensive full suspension bikes, but a rigid bike with the same specs would still be cheaper. On all of these if you find yourself riding on the street more than the trails you can always replace the knobby tires with some slicker ones to cut down on your rolling resistance.
Guess this all depends on how much you are looking to spend, what kind of riding you're expecting to do, and if you want to get a bike to "grow" into, or something cheap that you can replace later if you really get into biking.
Hope some of that helps.
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