Shiplap is a method of cutting out a 1/2 by 1/2 inch of wood from both edges of a 1x6 board. When nailed up the boards interlock and lay flat against the studs providing a solid flat wood surface. This is an old construction technique often found in the sheathing under the clapboards or shingles of houses. Clapboards are usually 1/2x4 or 1/2x6 tapered boards nailed up with the thicker edge overlapping 1 inch or so the thinner edge of the board below. Clapboards are usually the finished siding on houses. There are many variations to that presented above; hopefully that helps explain the difference?!
Thanks Don for your answer, it explained it well to me. I have an old store building. Part of the inside walls are tung and groove bead board, the rest if 6 inch horizontal boards. I wonder if they could be shiplap. Do you know a sight on the net that has a picture of shiplap. Thanks again you answer was very helpful to me. Debbie C
Debbie, I don't know of a site off hand. You might try searching "shiplap" and see if anything comes up. You could also go to your home center or lumberyard and they could show you a profile or example of the wood. Don