This is what I can surmise from the documentation (and is also in chronological order, earliest to latest):
- csc = C Sharp Compiler (from Microsoft)
- mcs = Mono Compiler Service (or potentially Mono C Sharp, but compiler service is spoken about elsewhere, and this compiler is actually written in C#).
- gmcs = Support for Generics
- smcs = Support for Silverlight (Moonlight)
- dmcs = Support for Dynamic
All the compilers also support additional things, but it appears as though the prefix of the compiler command-line name takes the dominant reason for the increase in version.