The IIS actions allow you to administer Microsoft IIS 5 and IIS 6 servers from your build. There are different sets of actions for IIS 5 and IIS 6.
The IIS 5 actions use ADSI (the Active Directory Service Interface) to connect to IIS. This means that only the currently logged in user's credentials can be used to connect to the server.
The IIS 6 actions use WMI (the Windows Management Instrumentation) interface to connect to IIS. This means that alternative credentials (ie username/password combinations) can be used to connect to IIS. In addition, there are some features which are only available in IIS 6. Generally, though, the two actions have identical feature sets.
Note that IIS 5 actions can still be used for connections to IIS 6, although IIS 6-only features will not be available.