If you mean the difference between Ubuntu and Ubuntu CORE, for example. Then the answer is that the latter manages the minimum required memory and processor functions sufficient to handle the inputs/outputs and hardware available. Ubuntu, on the other hand, runs many more programs that, while not indispensable, make the system take full advantage of the available hardware to provide a better user experience.

Embedded Linux is more focused on the processor-level operating environment in general. But it's already a lower level of programming.