Definitions:
From Boxes and Arrows: A wireframe...
- demonstrates a site concept quickly, allowing clients (stakeholders) to react to content placement and rendering
- can provide guidance to visual designers with respect to information priorities
- allows for usability testing early in the project lifecycle
- can elaborate on a singular vision for the site
- can facilitate collaboration between design team and information architects
- is easy for clients (stakeholders) to understand
The obligatory wikipedia definition:
A website wireframe is a basic visual guide used in web design to suggest the layout of fundamental elements in the interface. Because of this they are often completed before any artwork is developed. When completed correctly they will provide a visual reference upon which to structure each page. Wireframes also allow for the development of variations of a layout to maintain design consistency throughout the site. This is an important part of the initial development stage because it creates user expectations and helps to develop an awareness of and familiarity with the site.
How wireframes might look like....
Note that there's no general look of wireframes - while people agree on the purpose, the depth of detail varies a lot.