What is the difference between SLT & SLR cameras?

I'm totally confused about buying a new camera… My choice was nikon D5100. But someone inform me aboout the Sony SLT a57… SLT a57 drawing my attraction because of it's features and other thing… And D5100 doesnot have body focus motor thus lenses become expensive… Can you tell me which is better SLT or SLR? Which one to chose and why?

Nikon is the only maker that has this focus motor issue. Every other maker has the focus motor in the lens. So having a Nikon with no in-body focus motor is the same as having any other brand.

Why do you think the lack of focus motor makes lenses expensive for the D5100? The DX lenses are some of the cheapest in Nikon's range and they all work with the D5100. The only exception is the cheap old 50mm F1.8D lens, which won't autofocus with the D5100, and that's really designed to be the full frame standard lens. The DX equivalent is the 30mm f1.8, which is reasonably priced.

The SLR cameras have a mirror which flicks up when you take a picture. When not taking a picture the mirror reflects the image from the lens up into the viewfinder. With an SLT camera, the mirror is fixed, so some of the light passes through to the sensor and some is reflected up into the viewfinder.

The fixed mirror has the advantage of not moving which means you can get a faster frame rate per second - assuming your camera's memory buffer is big enough and the SD card writer is fast enough. As the mirror does not have to move you tend to get less shutter lag and the amount of camera shake reduces slightly. For sports photography the faster frame rate and reduced shutter lag could be useful, but it's marginal otherwise.

On the down side, with the SLT design, not all of the light passes through to the viewfinder, so either you end up with a less bright viewfinder or you end up having to use an electronic viewfinder, which is not as good as an optical viewfinder that you would get with a DSLR.

The other downside of a SLT is that you get less light reaching the sensor. That's about 1/2 stop, which means using slightly slower shutter speeds or slightly higher ISO or slightly wider apertures to compensate. And a lens with a wider aperture costs a lot more than the extra cost of a focus motor - compare the cost of a 50mm f1.4 and a 50mm f1.8 for example.

Personally I would go for the D5100 unless you are an avid sports photographer and want the extra frame speed. The brighter viewfinder is worth a lot and the D5100 really is an excellent camera.

If you value the ability to see exactly what the camera sees through the lens then the SLR is for you. If not, the SLT would become attractive. The SLT uses a translucent mirror for faster autofocus response. Translucent mirrors split the light passing through the lens passing most for the digital sensor and bouncing some to the autofocus module. In doing so, you lose (terribly) the ability to shoot with autofocus in low light conditions.