.net - C# using statement at the top of the code page -

almost c-sharp files have using statements @ top of page

i.e.

using system; using system.io;  //code.... 

what using statements mean @ top of page? why syntax different other using statement declarations. i.e.

using (resourcetype resource = expression) statement

those using directives. tell compiler namespaces in find classes use in code.

they different (and different) using statement defines scope disposable objects.