How IISview counts hits

A record in the web log (successful or not)
A record in the web log indicating a successful fetch (status code 200). (So files downloaded, not just checked against a cache).
Page is container document (e.g. HTML, PHP, ASP). A hit includes other content such as images, flash and javascript files. These included files are not counted as pages, only the container documents.

Unique pages
Unique pages are the "File" with any query stripped off. The number of distinct names (as distinct from number of hits). This is a clue as to how much of you site is being reached.

File type
The filename extension from each hit's filename after any query string or anchor tag have been stripped.
KBytes of traffic sent. IIS records the number of bytes sent back to the browser for each hit. (sc-bytes).
Unique/Distinct Vistors
A simple visitor is just a logged IP address. So users accessing via NAT will only get counted once. (e.g. 2 different PCs on a home network with one IP address on the Internet will be counted as one. If the home router reconnects with different IP addresses, these will count as different visitors.)
A session is a bit more complicated. It is is a unique (IP address + User agent string) combination, having a hit at least once every 20 minutes. IISView records the time and file when a combination is first seen. The time and file of this combination is tracked until there has been no requests for 20 minutes. The time between the first and last contact is the session time. Mean and standard deviation statistics are collected for the number of fetches and length of stay for each classification of stay.
Concurrent Visitors
Maximum concurrent visitor sessions.
Top Reader
Top readers are based on the top visitor IP addresses, and then the corresponding reverse lookup giving the hostname.
Response time (mS)
IIS reports the time taken for each hit. This response time is averaged (and peak measured) by day and by timeslot. So daily response time is the sum of all the response times for the day divided by the number of hits on that day. Timeslot is the sum over the month of all the response times in that hour (e.g. 13:00->14:00) divided by the number of hits in that timeslot.
Last field of the hostname
Generally last 2 fields of fully qualified hostname indicate the company or ISP for US based domains under .com and .net. Correction is made for countries that impose a country level high level domain e.g. in Australia or for the UK, whereas some countries do not do this under their country e.g. .it in Italy.
Entry/Exit page
In a session, the entry page is the first URL requested in the session, and the exit page the last (which is a bit unreliable as an indicator).
IIS can record the link the referred the browser to this web page. The referring site is the sitename part of the referrer (i.e. the page level detail of the referring web page is omitted).
Broken links
These are hits that resulted in an error status code. So it may be a broken link to the site from within the site, or from another site. The broken referrer references indicates the page that contained the broken reference. A "-" for the referrer probably indicates a robot or a probe searching for a known vulnerability.
This is the "agent string" provided by the browser or robot. These are then summarized in the "browsers and operating systems" table where the agent string is analysed to identify the general browser or operating system type.
Robots, a.k.a. spiders or crawlers, are identified in one of 2 ways. The first is by noting the agent string of any browser that is fetching the site's "robots.txt" file. The second way is identify well-known string patterns within the agent string. IISView collects some statistics separately for agents classified as not being a robot, such as sessions.
Bounce rate
This is the percentage of sessions that were less than 3 seconds compared to the whole.
Input filter
For a "site", a filter set controls which IIS log records are included in the statistics gathering phase. Each record is tested against the rules in the filter set and if the field value in the filter matches the filter pattern then the record is excluded if the rule indicates so.