OBW is a form of stack 'n whack. 6 or 8 layers of fabric are aligned so that the same design element is in the same place in each stack. If using 6 layers, you cut equilateral triangles (60 degrees at every corner) from the fabric after first cutting it into strips (strip length can vary). You then sew each triangles set into two half-hexagons and arrange them on your design wall, keeping each set together but not completing the sewing until the design is complete. (This eliminates sewing y-seams later.) Doing 8 layers is a little more complicated.
Sounds easy, is easy, but the book One Block Wonders by Maxine Rosenthal (or the sequel, One Block Wonders Encore) explains much, much more that will help to make your project a success. The beauty of the finished quilt depends largely on fabric selection (she gives guidelines for that) and placement on the design wall. She also gives helpful advice for that, and it's actually a lot of fun.
The sewing is not difficult, and the finished quilt can be amazing. I thought that the money for the book was well spent. Have fun making your own OBW, and be sure to show us what you've done.