One of the most important tasks as a webmaster is to monitor your log file data.
I ran into this excellent page detailing what each entry in a "raw" log file means:
Raw log file is the big file on your server that contains a line for each page or image requested by a visitor, it is the fie from which your statistics program derives its data.
Personally I use the free AwStats to monitor and generate log reports, it is popular but there are many other programs that others use as well. In many instances I find it very important to go dig out a specific entry in the log file to analyze it in detail. There is gold in them log files if you dig.