http://www.techradar.com/news/photography-video-capture/cameras/best-compact-system-camera-2013-the-best-models-reviewed-960832

Once upon a time, keen photographers bought a DSLR – it was the established order of things. But the mirror mechanism of a DSLR is complex and noisy and adds to the weight of the camera, and that's where the mirrorless camera, or compact system camera comes in. They keep the big sensors and interchangeable lenses of DSLR cameras but ditch the mirror to produce a smaller, lighter and simpler camera.

In fact, there are still pros and cons to both designs. If you want to find out more, read this: Mirrorless vs DSLR cameras: 10 key differences.