Hi guys, how to you read the log files from /var/log/messages?

i mean the datas and figures, what do they represents?
example

Jul 17 22:05:08 router dnsprobe[276]: Switching Back To Primary DNS server
Jul 17 22:26:11 debian -- MARK --
Jul 17 22:46:11 debian -- MARK --
Jul 17 22:47:36 router -- MARK --


cheers
nic