Mike C - "in 25 words or less..."

"N" or Normal development - time and temperature that will give the negative a "normal" contrast range, usually considered about 5 zones (stops), to print on your "normal" paper, usually Grade 2 or 3.

"N-" or Contracted - less developement (shorter time) to compress the contrast range. This technique takes a subject with a wide contrast range and compresses it to fit into the 5 zones on the film and therefore onto Grade 2 paper.

"N+" or Expansion - more development (longer time) to expand the contrast range. This is used for low contrast subjects, it stretches the subject contrast to fill the 5 zones on film and your normal paper.

Everyone has his/her own way of explaining this, a search for "zone system" will have a million and one results. If you want the complete explaination from the source, read "The Negative" by Ansel Adams. A warning - there is math involved!