I dug around a bit and found this:

http://shwinandshwin.com/2014/03/knits-102-cover-stitch-and-overlock.html
http://www.onelittleminuteblog.com/2013/01/serger-and-coverstitch-techniques-stretch-yourself/
http://fashionskyfall.com/sewing-machines-vs-sergers-vs-cover-stitch-machines/
http://fashion-incubator.com/flatlocking-compared-to-cover-stitching-and-overlocking/

 - The **cover stitch** is the double (or triple) stitch used for hemming knits or topstitching seams that need stretch.  The result is - double stitch on one side and the chain stitch on the back side which allows for the stretching of the seam. [Picture][1]

 - Serger stitch - wraps around the edge of the fabric and is invisible on the top side of the fabric.  [Picture][2]  It seems from what I'm reading to be synonymous with an overlock stitch - From (http://www.onelittleminuteblog.com/2013/01/serger-and-coverstitch-techniques-stretch-yourself/):

> OVERLOCK STITCH: This is the basic serger stitch. It can be sewn with
> 2-5 threads, but the most basic uses either 3 or 4. It is called an
> overlock stitch because the threads are cast over the raw edge of the
> fabric, locking it into a finish that won’t unravel or fray. When
> using a serger for basic overlocking, you can sew even when there is
> not fabric under the needles. This is called “chaining”. It makes it
> really easy, because you can put your foot on the pedal and start
> serging, feed the fabric through to create your seam, and then chain
> right off the end.

The last link suggests that a coverstitch machine does less than a serger.

I haven't gotten into buying a serger yet, but my quick poking suggests that a serger can be a somewhat generic name - and it's worth it to learn exactly what stitches - overlock, cover stitch, and flatlock - it does.  As ever, also, be sure to learn how it works for hems and blindhems.

  [1]: http://4.bp.blogspot.com/-bDF-XIvOYUQ/Uy5az_05KgI/AAAAAAAAOvQ/6GsfADQJbls/s3200/knit102d.jpg
  [2]: http://4.bp.blogspot.com/-ma3jIdGHxUY/Uy5a1_LBw7I/AAAAAAAAOvc/zSN21d4NTeM/s3200/knit102h.jpg