Apache can, and usually does, record information about every request it processes. Controlling how this is done and extracting useful information out of these logs after the fact is at least as important as gathering the information in the first place.
The logfiles may record two types of data: information about the request itself, and possibly one or more messages about abnormal conditions encountered during processing (such as file permissions). You, as the Webmaster, have a limited amount of control over the logging of error conditions, but a great deal of control over the format and amount of information logged about request processing (activity logging). The server may log activity information about a request in multiple formats in multiple logfiles, but it will only record a single copy of an error message.
One aspect of activity logging you should be aware of is that the log entry is formatted and written after the request has been completely processed. This means that the interval between the time a request begins and when it finishes may be long enough to make a difference.
For example, if your logfiles are rotated while a particularly large file is being downloaded, the log entry for the request will appear in the new logfile when the request completes, rather than in the old logfile when the request was started. In contrast, an error message is written to the error log as soon as it is encountered.
The Web server will continue to record information in its ...