1. As a directive(shortcut for namespaces):
System.Data.SqlClient.SqlConnection myCon = new System.Data.SqlClient.SqlConnection(connectionString);
To simplify the above statement just write:
using System.Data.SqlClient;
before any declaration(on very top of the class) and then you may write the simplified statement as:
SqlConnection myCon = new SqlConnection(connectionString);
This way you have included the namespace required for "SqlConnection" class so that you don't have to include the complete namespace whenever you create the object of "SqlConnection" class.
2. As a statement(to dispose any object references implementing the IDisposable interface)
using(SqlConnection myCon = new SqlConnection(connectionString)){
}
It is used to release resources automatically but can only be used with types that implement IDisposable.