Simple Explanation:
Every web page is made up of a number of items; the page itself, graphics and other things. If a page has 9 graphics and on one page, every time a person visits the page it will register as 10 hits (1 page plus 9 graphics)

Extended Explanation:
A hit is any response from the server on behalf of a request sent from a browser. This includes any response from the server, not only text files or documents. If, for example, a HTML page has two images embedded, the server generates three hits if this page is requested: one hit for the HTML page itself and two hits for the two inline images.

Simple Explanation:
This is the number of web pages viewed on the web site. If you visit a website and click on the contact page, then the information page and then the about us page it will count as three page views.

Extended Explanation:
Pageviews are all files which either have a text file suffix (.html, .text) or which are directory index files. This number allows to estimate the number of "real" documents transmitted by your server. If defined correctly, the analyzer rates text files (documents) as pageviews. Those pageviews do not include images, CGI scripts, Java applets or any other HTML objects except all files ending with one of the pre-defined pageview suffixes, such as .html or .text.

Simple Explanation:
The total amount of data, measured in kb, sent to all users.

Extended Explanation:
This is the amount of data sent during the whole summary period as reported by the server. Note that some servers log the size of a document instead of the actual number of bytes transferred. While in most cases this is the same, if a user interrupts the transmission by pressing the browser's stop button before the page has been received completely, some servers (for example all Netscape web servers) do not log the amount of data transferred but the amount of data which would have been transferred if the user would have completely loaded the page.

Simple Explanation:
The number of unique different visitors you have based up one day. If you visit a site and see any number of pages on the site it counts as one session. If you visit the site later in the day, from the same computer, and look through more pages it still only counts as one session total. If you visit the site the next day from the same computer, it will add another session to the total count.

Extended Explanation:
This is the number of unique hosts accessing the server during a given time-window. This time-window is one day by default. For example, if the time-window is two hours, all accesses from a certain host in less than 2 hours after the first access from this host are lumped together into one session. All following accesses more than 2 hours apart from the first access will be counted as a new session. This way you may get an estimated number of how many sessions.