Mirrorless cameras are the latest type of cameras that are becoming a stronger competitor with DLSRs. DLSR stands for digital single-lens reflex, which means that as light is poured into the camera through the lens, a mirror will pop up out of the way when a photo is taken. The light hits a digital sensor for a manually controlled amount of time, and the photo is created. When not taking a photo, the mirror directs the light from the camera up into a prism, which spits out a preview of the image through the viewfinder. The mirror also directs part of that light to a dedicated, separate sensor for autofocusing. Mirrorless, on the other hand, is completely digital. Light flows through the lens, but instead of hitting a mirror, it hits the sensor constantly. The image seen in the viewfinder is electronically transmitted from the main sensor and there is no separate, dedicated autofocus sensor.
It is still being debated whether or not this method takes down the quality of the process when compared to DLSRs. By not having a mirror, these cameras become lighter and smaller than their rival, but due to everything being electronic, their battery life is not as good. As for performance, DLSRs work better in low light while mirrorless have the advantage of better video quality. Another advantage of a DLSR is the selection of accessories- many more lenses, microphones, etc are available for these types versus the newer mirrorless cameras.