Generally speaking, "Views" is the number of times somone viewed a post on the web via a browser, and "AggViews" is the number of times someone viewed the post via the RSS and Atom feeds.
More specifically, the web view count is only updated in the EntryView control. This is displayed when you are viewing a single post. If you are viewing a list of posts, such as on the blog home page, the view counts of the posts are not updated.
The AggViews works by embedding an "AggBug" invisible image into each post in the RSS/Atom feeds. When someone views the posts in a feed reader that allows images, the image pionts back to a handler on your CS site that updates the AggView count for that post.
HTH