To Simply put, a mirror less camera lacks mirror for the view finder.

Unlike a Digital Single Lens Reflex (DSLR) camera, lacks a mirror that typically allows the photographer to view the scene through the lens using an optical viewfinder. When developed, the mirror approach was a great way to frame your photos before you pressed the shutter button. Now, using digital viewfinders found in mirrorless camera systems you no longer need the mirror apparatus to frame your photos. You can just view the scene directly off the camera’s sensor. This is a fantastic improvement over that standard DSLR approach for several reasons; no mirror means the camera can be smaller, using a digital viewfinder means what you see is much closer to how the final photo will look when taken. Both are really good improvements, and are clearly better than the classic DSLR, but all is not well in mirrorless camera land.

 we should not forget about the advantages of DSLR cameras for specific situations such as sports and wildlife photography, where high ISO performance and autofocus speed/accuracy are critical. I just don’t see manufacturers making compact 600mm lenses anytime soon that can focus as fast as current professional lenses. I am not saying that it won’t ever happen, but it will take us a long time to get there.

Perhaps the logical answer to this entire mirrorless vs. DSLR fiasco, at least for Canon and Nikon, is to offer a hybrid viewfinder system where we can switch between an OVF and EVF, much like the FujiX series, so that we can have the benefit of a real time, no delay real world view, as well as a preview of our shot so we can adjust exposure and white balance before hitting the shutter.