This is a T-Ring.  One side features the lens mount appropriate to your camera, the other side features a standard female "telescope thread" which is used to attach...

...a camera adapter.  In this picture you see the T-Ring threaded on to the left end and the standard SCT rear cell fitting on the right end.  To attach this to the telescope you remove the visual back and replace it with this.  E.g.:

The whole thing looks like this when assembled.  The light that comes down from the secondary goes straight back to the film plane.  This is basically a 1250mm camera lens (about 25x).

If you need more magnification, you need an extension tube.  An extension tube is just like is sounds - it moves the camera back so you can put an eyepiece in front of it, like this:

Here is the eyepiece inserted in the visual back.  Next you screw the extension tube into the T-Ring, attach the T-Ring to the camera, and slide the whole thing over the eyepiece and visual back:

A pair of set screws holds the assembly in place.