So what is a kipu ?

Currently, most investigators consider the kipu to have been the 'written' language of the Inca people. The kipucamayocs were the database administrators, so to speak, and were the only ones capable of interpreting the meaning of the knotted strings, based on the color (e.g. yellow for gold, red for army), positioning and type of the different knots.

The knots were made in a base-of-10 positional notation that even shows the use of the zero and they could mean anything from the number of babies born in a village to the amount of corn that was distroyed in a hail storm; all depending on the context they were used in.
Some scientists believe they were also used as memory aids for storytellers or poets.

Sadly enough however, since the kipus were closely related to the Inca culture and religion and because the Spanish conquistadores presumably did not understand much of it, they were considered a threat and therefore erradicated as thoroughly as possible.
This may mean that a part of the knowledge about kipus will forever be lost in history.
More kipu information
Although the kipus have had a rough time, on this site, you can still view some examples.
First hand account
Denmark's Royal Libary put Guaman Poma's "El primer Nueva corónica y buen gobierno" online.
This manuscript was written in 1615 by a native Andean and describes the Inca empire, illustrated with drawings. You can find his drawing of a kipu here.
A number of links
For those who would like to continue investigating, more info is available at National Geographic or the Khipu Database Project by Gary Urton at Harvard University.
Also, compiled a complete list of Inca related links.