Linux is highly supported on different architectures like ARM, MIPS, x86, m68k. When some refers to a target platform is combination of an architecture (CPU) and a board that holds that CPU. Selecting the right platform depends on the application requirements or your actual needs.
If you are in the case as me (learning purposes) you can choose any know handled device that supports Linux like N800 Nokia tablet PC, Ipod player or if you can’t afford any of this gadgets you can use Qemu for target virtualization.
My choice is Qemu targeting an ARM achitecture.