There certainly can be differences between NEF and JPG images - but they tend to manifest themselves in more challenging conditions, or when performing significant post-processing.
The first major difference is that a JPG has only 8 bits per channel, rather than the 12 or 14 bits of NEF files. As a result, NEF's can maintain fine gradations of tone in highlights and shadow areas much more readily.
The second thing to remember is that JPG is always a "lossy" format, because of the data compression applied. Whenever you open, edit and then re-save a JPG, image data will be lost. After a few iterations, you'll start to see big differences.
Having said all that, today's cameras have ever better JPG engines, and I'm quite happy to use that format for "holiday snap" type of images, or if I know I want quick results out-of-camera when shooting in kind lighting conditions