A file that records the activity of a web server.

Log files can include this detail for each request:

date and time of the request

location of resource requested

server response to the request

IP address of requesting computer

user agent of requesting browser

referral string of requesting browser

...in addition to some more technical information.

Not every web server will be setup to display this information in exactly the same order. Look to the top of the log file for a line with "column" headings.