Properties in Continua CI serve two purposes, they describe the location of tools on the server and the agents, and they also serve to let the server know about the agent's capabilities. For example, if you have a Stage on your Configuration that uses the NUnit action, then that Stage requires NUnit to be installed. When the server is selecting an agent, it looks for an agent that has a property matching the version on NUnit your action needs.
Property collectors determine how the server or agent finds a tool. The default property collectors describe the locations where the agent might look to find a tool, so for example the NUnit 2.4.8 property collector tells the agent to look for a file named nunit-console.exe in either %PROGRAMFILES%\NUnit 2.4.8\bin\ or %PROGRAMFILES(x86)%\NUnit 2.4.8\bin\ or on the %PATH%. If it finds the file there it will report a property nunit.2.4.8.path, if doesn't find the file then it will not that property.
You can override the default property collectors, or add new ones. Property values can also be manually overridden on an agent by agent basis.