I tend to use structs only when there is one method is needed thus making a class seems too "heavy". Thus sometimes I treat structs as lightweight objects. BEWARE: this does not always work and is not always the best thing to do, so it really depends on the situation!
For more formal explanation, MSDN says: http://msdn.microsoft.com/en-us/library/ms229017.aspx
Also This link verifies what I mentioned above, structs should only be used for housing a very similar question posted already:small amount of data. http://stackoverflow.com/questions/85553/when-should-i-use-a-struct-instead-of-a-class