If you are talking about the machine quilting part of the quilt-making...
If I want the thread on the back to not be visible, I use Superior Bottom Line thread...it is very thin and just blends into the back of the quilt. It is a 60 wt. thread. I then use reg. machine quilting thread on the top, which is usually a 40 or 50 wt.
If I want my quilting to stand out on the back, I use the same machine quilting thread in the top and the bottom. (40 or 50 wt.)