The term "unmanaged resource" is usually used to describe something not directly under the control of the garbage collector.
For example, if you open a connection to a database server this will use resources on the server (for maintaining the connection) and possibly other non-.net resources on the client machine, if the provider isn't written entirely in managed code.
This is why, for something like a database connection, it's recommended you write your code thusly:
using (var connection = new SqlConnection("connection_string_here"))
// Code to use connection here
As this ensures that .Dispose() is called on the connection object, ensuring that any unmanaged resources are cleaned up.
Check this link http://stackoverflow.com/questions/3607213/what-is-meant-by-managed-vs-unmanaged-resources-in-net
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