If you know MRTG, you can think of rrdtool as a reimplementation of MRTGs graphing and logging features. It describes itself as 'magnitudes faster and more flexible than you ever thought possible'.
The best front end for the job seems to be Cacti, though you can make MRTG use rrdtool as a backend.
See RRD for more information on round-robin databases
6 pages link to rrdtool:
