You can either copy and paste XML files into the text area, or you can upload your files.
If your uploaded files contain references to other files, you will be prompted for those. During your session, all of your files are kept for you on the server, and you can edit each of them.
If errors are found in any of these files, they are marked in the text with icons. If you point at one of these icons with your mouse, the error is desribed in a popup window:
If you would like to see how xmlvalidation.com works, but don't have an XML file to use for testing, you can use this example.
First, paste or upload the XML file. You will then be prompted for the XSD file. When both are uploaded, the errors will be highlighted and described.
You can read more about XML on http://www.w3.org/XML/.
More tools for web development can be found on uitest.com.