It is pretty easy to gather status information from all sorts of things,
ranging from the temperature in your office to the number of octets which
have passed through the FDDI interface of your router. But it is not so
trivial to store this data in a efficient and systematic manner. This is
where RRDtool kicks in. It lets you log and analyze the data you gather from
all kinds of data-sources. The data analysis part of RRDtool is based
on the ability to quickly generate graphical representations of the data
values collected over a definable time period.

WWW: http://www.rrdtool.org/