Working with file paths in classic ASP was a bear - you had to do a fair bit of string manipulation to find out common tidbits of information about a file path, such as the file's extension, the directory the file resides in, the file name, and so on. Fortunately, with ASP.NET things are a lot simpler thanks to the System.IO namespace's Path class, which contains numerous static methods for performing common file path tasks.
The Path class contains methods for combining a directory and filename into a single path, determining the extension of a file, grabbing just the directory or just the file name from a fully qualified path, determining if a path has a rooted directory or not, and so on. For a complete list of Path class members check out the technical docs. For a more readable explanation of some of the Path class's more useful methods, check out the "Working with File Paths" section of the article Little Known, Invaluable Methods and Properties in the .NET Framework Base Class Library.