Negative space is the space within a photograph which is left unused. Negative space is a great way to help add emphasis to your main subject. Negative space is generally just one color (or black or white) and takes up a fairly large portion of a photograph. Using a simply background such as a clear sky or a white table are two good examples of items which could produce negative space.

However, negative space doesn't need to be a solid color. Negative space can also be a textured color with various degrees of details. For example, in the picture below the negative space is the field of wheat before the farm.

Other photography pages of interest:
High and low key photography.
Using rhythm and pattern in photography.
Golden ratio.

Other photography resources:
More photography resources
Photography Glossary.