Is a Through-the-lens camera the same as a SLR one?

Sort of ... In addition, you would include TLR (Twin Lens Reflex).

It is through the lens viewing camera vs. a viewfinder camera.

er, yes, with an SLR (single lens reflex) camera, you see in the eyepiece the light that is coming through the lens, reflected off a mirror and sent to the eyepiece through a prism.
When you shpoot the pic, the mirror flips up and the shutter opens, exposing the film.

You could describe this as "through the lens"

The other type of camera, is a"rangefinder". this type used a corrective lens next to the camera lens to give you an approximate view of what the lens has framed.