The idea is simple: to build an universal abstraction layer on top of other existing libraries, that developers can use in their code instead of the platform-specific calls, to make it possible for an application to get build-time support for a major number of previously incompatible platforms.

The list of supported (or planned) backend libraries follows:

SDL 1.2 (WIP)
SDL 2.0 (WIP)
NDS
GBA
NES

Example programs (and makefiles) will come soon.