From MSDN:
C#, through the .NET Framework common
language runtime (CLR), automatically
releases the memory used to store
objects that are no longer required.
The release of memory is
non-deterministic; memory is released
whenever the CLR decides to perform
garbage collection. However, it is
usually best to release limited
resources such as file handles and
network connections as quickly as
possible.
The using statement allows the
programmer to specify when objects
that use resources should release
them. The object provided to the using
statement must implement the
IDisposable interface. This interface
provides the Dispose method, which
should release the object's resources.
In other words, the using statement tells .NET to release the object specified in the using block once it is no longer needed.
