When you first install a Munin node it will try to install as many plugins as it can so that it can report on different things. For example, if you have a Varnish server running then Munin will detect this and enable the plugins so that it can report on the activity of Varnish. Once you have started getting data through to your Munin server then you can turn on plugins on the nodes to get more data.
When Munin does a data update it stores all of the data from the nodes as a set of rrd files. These files are then picked up by the munin-graph and munin-html programs and turned into the graph images and web pages that you are probably familiar with if you use Munin.
I have been searching for a good server monitoring solution for a while so that I can keep an eye on some of the servers that I run. Tools like Smokeping, Cacti and Nagios seemed promising at the outset, but they are more concerned with bandwidth and server status, not how the server is running.