How IISview counts hits

Hit
A record in the web log (successful or not)
File
A record in the web log indicating a successful fetch (status code 200). (So files downloaded, not just checked against a cache).
Page
Page is container document (e.g. HTML, PHP, ASP). A includes other content such as images, flash and javascript files. These included files are not counted as pages, only the container documents.
KBytes
KBytes of traffic sent (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.)
Session
A session is a bit more complicated is is a unique (IP address + User agent string), having a hit at least once every 20 minutes.
Concurrent Visitors
Maximum concurrent visitor sessions
Top Reader
Top readers are the top simple visitor IP address.
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.
Response time (mS)
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 numner 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.
Country/Domain
Last field of the hostname
Company/ISP
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. .com.au in Australia or .co.uk 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).