Neng or ke yi?

I am a newbie and I don't even really know the characters well yet for each of these words, but I was wondering if you could lay out how to use the different words "neng" (to be able to) and "ke yi" (can, or to be able to). They are nearly identical in meaning, though don't seem to be used interchangeably.

Please let me know. 

Peace,
Darian