1. Hits - this simply refers to the number of 'elements' loaded on your site. If one page has five images in it, viewing that page once adds 6 hits (one page + five images).
2. Impressions - the number of times all the pages on your site are seen (also simply called pageviews). Impressions are sometimes referred to as 'hits' which can cause confusion.
3. Uniques - the number of people that visited your site.