Hits: These are number of objects that have been transferred to your
visitor’s web browser. So if your page has 10 images on and they are
downloaded at once then you will receive 11 hits (1 HTML page + 10
images).
Impressions/Pageviews: The number of pages that were viewed by your visitors.
Unique Visitors: The number of people that visited your site.
Thus if someone visits your site and go to 3 pages, each of which has 5 images
on it your website statstics will report the following: 18 Hits; 3
Impressions; 1 Unique Visitor.