It is a 24 megapixels digital SR camera. Sadly to say that I do not use this maximum resolution, because files are just too big. I even skipped using NEF raw files they are just enourmously huge for me. You can choose between 6 MP, 16MP and 24MP. I take pictures at 6 MP and select 24 MP only shooting far objects with telephoto zoom lens in order to be able to crop it. This camera has lot of pro features.
It can take pictures quite fast, I think something like 4 frames per second. It is just my perception, specification might say something different. It has live view and ability to shoot motion picture at pretty decent resolution 1080p.
D7100 has top LCD with runtime parameters. It can be lighten by moving power know right far most. It means more battery life because there is almost no need to use big screen. Viewfinder is big which is good for me, I just like that way. There are few more interesting things worth mentioning like HDR (multiple exposures, automatic lighting correction (within single frame). There is somehow funny feature of additional crop put atop of already cropped sensor (comparing to 35mm equiv.). It might be useful if you are only interested in mid section of the picture and would like to have telephoto using normal lens.
I use it with 50mm AF Yongnuo f/1.8 lens which was 60 EUR brand new. It is quite good betweem f/2.5 and f/3.5, good at f/4, very good at f/5.6. Going further up with f-stop numbers you will likely see distortions all other bad things as usual. My D7100 is far better than D70s and D200 in terms of dynamic range and signal-to-noise ratio. It is said that D70s is best at most at ISO 500, D200 at ISO 600 and this one is good even at ISO 1600.
Camera has CMOS sensor as D200. D70s has CCD sensor. I read somewhere that there is no difference in terms of picture quality between these two most common technologies. The difference is in materials, filters and software. And it is. I can say that pictures taken with this camera will be similar to that taken with D200. Differences between D70 and D200/D7100 are minor but noticable at high magnifications. Especially in terms of noise.
Top picture comes from D70s and NIKKOR 50/2. Bottom one from D7100 and YN50/1.8. There is a very big difference at book background. This color artifacts make pictures taken with D70s to look little posterized. Newer cameras like D200 or D7100 outputs image which is more clear and free of such noise. Of course this is important at low light conditions and not full sun outdoors shooting – at such conditions you can shoot with almost everything.
My D7100 has already taken over 300 000 pictures. I think the risk is high, but price was low (100 EUR). I think that I will take some 10 000 picture and pass it on to someone else. It is a very nice camera. I strongly recommend it.