man sw_vers | tail shows you the file you need to look at, namely:

/System/Library/CoreServices/SystemVersion.plist
/System/Library/CoreServices/ServerVersion.plist

When you cat those files, you learn what you need to know.

You have a few options:

sw_vers -productVersion 

system_profiler SPSoftwareDataType

Either will do what you need, and will have an output format that's parseable (if that's what you're after).

sw_vers

My suggestion is to use sw_vers. Example output as of 10.6.4:

> sw_vers 
ProductName:    Mac OS X
ProductVersion: 10.6.4
BuildVersion:   10F569

The answer that suggested system_profiler | grep 'System Version' is what I have tried to use in the past, but it has 2 problems.

1. It is slow since it generates a full system_profiler dump of the machine, gathering all hardware and software inventory information.
2. The output of system_profiler has changed over time. e.g. output of grep for 'Serial Number' on 10.6.4 is "Serial Number (system): ZNNNNNZNZZZ", whereas on 10.4.11 it was "Serial Number: ZNNNNZNZZZZ" - importance being the parse-ability of the output and the add " (system)" piece can be problematic unless you are expecting the change.

You can read the SystemVersion.plist on that particular volume-

/Volumes/<volume_name>/System/Library/CoreServices/SystemVersion.plist 

Go to

C:\Windows\System32

read the "license.rtf" file

I use the program Mactracker when I want to know about specific machines throughout Apple's History. With your model number you can identify all there is to know about that mac you have.

It is available form the App Store for both Mac and iOS devices.