RRDTool is really a backend tool. It is designed to store and display time-series data. It has a compact format for data storage and can create graphs based on this information. It is intended to be called by a frontend of some sort.
RRDTool is written in C. It has a perl module and a shared library to call it, or it called from the command line. To get really useful output, you need one of the many available front ends.
If you have an suggestions, comments, or corrections, please let me know.