Rest is a way of doing communication over the internet. It is a very basic process of picking addresses to serve as method locations and returning HTML standard data (javascript, css, html of course).
WCF is a .net library used to have two programs talk to each other using SOAP. Which consists of two very familiar programs trading class info.
Seeing as Rest is a process, and WCF is a class library, a better question might be “Rest vs Soap”.
The bottom line is, if you need two apps to talk, you might want to use WCF. Even if the apps are not both written in .net. However if you need information to be accessed by web tech(usualy javascript access is done this way) you’ll want to use Rest.
Just a quick side note though, WCF does Rest well too, so you realy can’t go wrong there.