The Buttonhole Stitch is nothing more than the Blanket Stitch stitched close together, with no fabric showing between the stitches. Although it can be used as a decorative stitch, it is most often used as an edge binding stitch (particularly for - yes, you guessed it - buttonholes).

Usually, the length of the stitch (the vertical part, in this drawing) is quite short. This makes a small, tight, bound edge.