Interface files are used to display windows in your script, they act like resource files adapted for Messenger. They are not particularly complicated to understand, however, they contain a lot of different kinds of objects and attributes and some of them will not even be of any use to your scripts. The end result will be worth the trouble though as you won't need to create external DLLs to display your windows and you'll benefit from the current look of Windows Live Messenger as well as possible future updates. Your script will be guaranteed to appear just like any other Messenger Plus! window.
The Schema file for Interface Windows can be temporarily placed in your script's directory to validate your XML files. However, the schema must not be redistributed with your script (it would only be dead weight anyway). Here is how to write the first element of your XML file to make your XML editor aware of the schema file:
<Interfaces xmlns="urn:msgplus:interface" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:msgplus:interface PlusInterface.xsd">
<Window>...</Window>
</Interfaces>
It is good practice to always link your xml files to their respective schemas to prevent any possible run time error caused by an error in your XML. XML files tend to grow rapidly and you'll soon be thankful for the validation layer brought by the schema file.
The best examples you can get for interface window files are probably the windows of Messenger Plus! itself. Those files can easily be extracted with a resource viewer tool such as Resource Tuner. Just open the "MsgPlusLiveRes.dll" file you'll find in the main directory of Messenger Plus! (by default: "C:\Program Files\Messenger Plus! Live") and get the files from the "Interfaces" section. It is important to remember that the interface files were created for both Messenger Plus! and the scripting system. This means that some attributes or elements may prove to be useless for your scripts. Additional support for missing pieces are added on a per-request basis.
If your XML Editor does not come with graphical browsing of the schema file, you can view a documentation generated by XMLSpy in the Interface Windows Schema Documentation (some information may be missing in this documentation). Some elements of the file are also explained in details in Windows for your Script.
Interface Window Schema File, Schema Documentation, Examples.