Access:
One line of your web server's log file. (A request for a single document or other object on your web server. Note that images count as documents in their own right as far as your web server is concerned.)

Hit:
Same as access

Visit:
A visit to your web server by one person, involving one or more hits over a period of time. Visits are estimated by examining the length of time between hits from the same address on the Internet.

Unique sites served:
Total number of individual sites (addresses) on the Internet which have accessed your web server one or more times during the interval covered by the report.

Unique documents served:
Total number of individual documents (files) on your web server which have been delivered one or more times during the interval covered by the report.