This is an interchangeable lens camera where you see an optical image in the viewfinder showing what the lens sees. They do this using a mirror inside the body that reflects the image seen by the lens up into the viewfinder. When you take a picture, the mirror flips up out of the way so that the image passes through the body to the sensor at the back of the camera. DSLRs work in the same way as SLR (single lens reflex) film cameras, but substitute a digital sensor for the film.
The big debate these days is whether modern mirrorless cameras have made the DSLR obsolete. Mirrorless cameras are mechanically simpler and smaller and have made many innovations possible, including advanced video capabilities and autofocus features.
In principle, many of the advances and features in modern mirrorless cameras could also be applied to DSLRs. In practice, camera makers have abandoned DSLR development in favor of mirrorless cameras, so we will probably never know.