A server log is a file (or several files) automatically created and maintained by a server of activity performed by it.

A typical example is a web server log which maintains a history of page requests. The W3C maintains a standard format for web server log files, but other proprietary formats exist. More recent entries are typically appended to the end of the file. Information about the request, including client IP address, request date/time, page requested, HTTP code, bytes served, user agent, and referer are typically added. These data can be combined into a single file, or separated into distinct logs, such as an access log, error log, or referer log. However, server logs typically do not collect user-specific information.

These files are usually not accessible to general Internet users, only to the webmaster or other administrative person. A statistical analysis of the server log may be used to examine traffic patterns by time of day, day of week, referrer, or user agent. Efficient web site administration, adequate hosting resources and the fine tuning of sales efforts can be aided by analysis of the web server logs. Marketing departments of any organization that owns a website should be trained to understand these powerful tools.

If you have acces to the server you can try to modify them or clear them but you must know where are stored on the server.

Source: wikipedia.com