While they are both increases, they are done differently. Using one instead or the other could mess up your total stitch count at the end of the row, expecially when the pattern specifies a particular one.
M1 is done between 2 stitches. kfb uses a stitch and adds one at the same time.
There are videos for them both on the video page, there may be vidoes of them on the Glossary page, too.