SHELL

pam_unix(cron:session): session opened for user root(uid=0) by (uid=0)

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.