DSLR's use a mirror in the body to reflect the image up into the viewfinder. The instant you take a picture, the mirror flips up and out of the way. Mirrorless cameras have no mirror. The image passes straight through to the sensor at the back of the camera which passes a live 'feed' to an electronic viewfinder.

Mirrorless models are more restricted, offering access to a small number of lenses from the camera maker, though the selection is growing. As they have been around longer, DSLRs tend to have a better selection of other accessories, such as speedlights (flashes). The difference is especially stark among traditional camera makers.