Ripping is for when the mistake is too far down in your knitted piece or too tricky for you to be able to fix without pulling all your stitches out to below the error.
Basically ripping involves removing the needles from your work and unravelling as many rows as you need to to so that you can undo the error and start restart you knitting at a point before you made the error. Once you have unravelled enough rows to get just below where the error was in the fabric, you pick up all of the stitches (put them all back onto your needle) and start knitting again.
Frogging also involves unpicking your work to before the error, but with this method you unpick your knitting one stitch at a time. I use this method when the error or mistake is on the same row that I am working on or on the row below.
To 'frog' a stitch you inset the working needle into the stitch below the stitch on your left hand needle and then slip the upper stitch off the needle. If you give the yarn a slight tug the stitch will unravel leaving the stitch from the previous row on your working needle.