Transaction.Commit() and Transaction.Rollback()

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Transaction.Commit() , Transaction.Rollback() ,
cmd.BeginTransaction(), cmd.EndTransaction()

Can any one explain what are the kind of scenarios that these set of
commands are being used. MSDN does have the code but does not explain
how are they different from simple ExecuteReader() or executing any
simple query.

Referring to any useful link, would be appreciated.

Regards

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