What is the difference between serger, coverstitch, and overlock machines?
I dug around a bit and found this:

From Knits 102:


  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.


Serger stitch - wraps around the edge of the fabric and is invisible on the top side of the fabric.




It seems from what I'm reading to be synonymous with an overlock stitch - From OneLittleMinute Blog:


  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.


Fashion Incubator 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.