Hit

An HTTP request made to your server. The request is not only for the HTML files, but also for the images, movies, and java script or css file which make up each web site page. If you use frames, then one actual page view can result in multiple hits, as the page is made up of multiple files. When log analysis programs read the server log files, they'll report total hits. This total will always be larger than the total page views.

Impression or Visit:

A page view. An impression is when someone views one of your HTML pages. The MOST IMPORTANT metric for measuring website traffic. 1 Visit = 1 Person

Referrer:

A page that links to your site. Looking at your referrers will tell you who's linked to your site. It can be another web site or a search engine.

User Agent:

This refers to the software used to access your site. Can be a browser, a client, a PHP script, or a search engine spider.

Counters / Trackers:

A counter is a simple script that records the number of visitors to a site and displays the total on the web site. Hit tracking software is preferable to hit counters because of the level of detail tracking software provides.