TTV stands for Through the Viewfinder and it is a technique where a photo is shot with one camera through the viewfinder of a second camera. Typically this is done with a digital SLR (I use a Canon) shooting through the viewfinder of a twin-lens reflex (TLR) camera. (I use a vintage Argus.) I shoot through a blackened tube connecting the two cameras — this helps eliminate ambient light and prevents reflections.
If you are interested in learning more about it, I suggest starting here with Wikipedia.