Digital Single Lens Reflex (DSLR) cameras use relatively large image sensors which improve the image quality of photos. I wrote previously about the importance of image sensors here. In a traditional DSLR, a mirror reflects the image your lens is capturing up and in the view finder for you to see. When the shutter releases, the mirror flips up and exposes the image sensor to the scene coming through the lens.
Mirrorless cameras have risen in popularity and challenged the status quo. In a mirrorless camera, there is no mirror (obviously) which means that the image coming from the lens falls directly upon the image sensor. Removing the mirror and the mechanics necessary to expose the image sensor has reduced the size of these cameras considerably. This is a huge perk for anyone who wants a more compact camera to take around.
Within the mirrorless camera subject is the micro four thirds standard. Lenses for cameras using a micro four thirds standard enjoy the freedom of freely interchangeable lenses. This means that a camera is not restricted to the manufacturer’s mount for lens selection. A Lumix micro four thirds camera can employ an Olympus lens. In contrast, a Nikon will only work with F-mount lenses.
Every camera is different and captures data in a unique way. There is no single, accepted best practice when it comes to capturing an image. In some cases, a mirrorless and SLR camera are completely and totally interchangeable. Arguments on one side or the other verge completely on academia without real world application. It is up to you to determine what your shooting requires and get a camera that matches best with your needs.