Hits - A hit is a request for any component of a website. If a page has 5 images then when that page is viewed it counts as 6 hits, one for the page itself and one for each image that appears on the page. This fact makes hits a poor judge of website traffic.
Impressions - This is simply how many times an ad has been asked for. Unfortunately this can be misleading due to browser caching. If a person returns to a page they have recently visited, chances are the images will not be asked for because the browser has saved a copy of that image and is using it to increase the load time. That is why the pageviews number may differ from the impression number of ads that appear on those pages. Because of caching images, pageviews are a more accurate count of the frequency an ad is viewed.
Pageview - This is the amount of times a webpage (a document ending in .html) has been requested. Since pages are less often cached, they are a better measure of activity.
Unique Hosts - This describes the number of individual machines which requested information from a website (usually measured monthly). This is the closest metric a website has to circulation, although it is possible with a large ISP such as AOL that many users are connecting to a website through the same machine. This is also true with a corporate multi-user connection which uses a firewall machine through which all of their employees are connected.
Visits/Sessions - A Visit (also called a Session) is defined as a consecutive sequence of requests from a unique host separated by 30 minutes. If someone visits a website they will be making requests for pages and images from that website. If they are inactive for over 30 minutes and then make more requests, their activity is counted as two visits. This is the standard set and used by Nielson Data Research.