RRD Tool is a C program written by Tobi Oetiker, which does all the hard work for Cricket. RRD Tool takes care of storing and normalizing the data after the collector fetches it, and draws the data onto graphs when the grapher asks it to. Without RRD Tool, Cricket would be useless.
Like Cricket, RRD Tool is distributed under the GNU General Public License. Tobi has asked authors of frontends (like Cricket) to also include an RRD Tool logo and a link on all pages generated using RRD Tool. The Cricket sample-config tree ships configured to put this logo on the bottom of all the pages created by the grapher. Please respect Tobi's wishes and leave the logo on pages that you configure. The logo looks like this:
There's more information available about RRD Tool from its homepage at http://www.rrdtool.org/.