Negative space or "white space" refers to unused white space showing through a logo. Strictly speaking it's not really unused, but an area that the designer chose to leave white.
In the example to the right, the designer combined a cross and a running man, but the running man consists of, uhm, holes in the cross!
That's an example of macro negative space. Micro negative space refers to smaller areas of white space, for example the gaps between the letters (called kerning).
Thanks to Biz-Logo.com for the example.
Further Reading:
Article on white space; http://www.alistapart.com/articles/whitespace
