AssemblyInformationalVersion and AssemblyFileVersion are displayed when you view the "Version" information on a file through Windows Explorer by viewing the file properties. These attributes actually get compiled in to a VERSION_INFO resource that is created by the compiler.
AssemblyInformationalVersion is the "Product version" value. AssemblyFileVersion is the "File version" value.
The AssemblyVersion is specific to .NET assemblies and is used by the .NET assembly loader to know which version of an assembly to load/bind at runtime.
Out of these, the only one that is absolutely required by .NET is the AssemblyVersion attribute. Unfortunately it can also cause the most problems when it changes indiscriminately, especially if you are strong naming your assemblies.