Assuming you have not changed anything from the default cron setup, this is your /etc/crontab running. On my Ubuntu 10.04.3 LTS server, its contents include:
# m h dom mon dow user command
17 * * * * root cd / && run-parts --report /etc/cron.hourly
So, cron wakes up every hour and runs any scrips located in /etc/cron.hourly. You probably don't have any which is why it doesn't do anything. It simply runs a root session which executes run-parts and then closes the session again.