Total Posts: 796
the log files are just that, logs of visitor activity on your site.
1) log_url: this is a table that tracks visitor(id)-> site url(id)->date/time
2) log_url_info: this is the table that maintains the unique ids for log_url, specifically it maintains id(autoincrimenting)-> site url(path) -> referrer url(path). You will get a lot of these, one for each unique combination of site and referrer url.
3) log_visitor: this is the table that tracks user specific activity id(autoincrimenting)->sessionid->first visit->last visit->lasturl(id)->store(id)
4) log_visitor_info: this is the table that tracks user specific attributes: id(autoincriment), referrer(url), useragent, charset, language, server ip, client ip
This provides a ton of historical analytic about your site and its visitors. Perhaps you are tracking this elsewhere such as google analytics, if so, I would recommend (instead of manually clearing the tables) just enabling the log cleaning option in administrator
http://mydomain.com/magento/index.php/admin/system_config/edit/section/system/ -> log cleaning
and setting the number of days to something your comfortable with (default is 180 days)