Custom Transaction codes

If we develop some Module pool programs or Report Programs, to make these programs available to the end users. we create transaction codes for these programes."se93" is the transaction where one can create his own custom Transaction codes.
Custom Tcodes will start with Y* or Z*.
For performing any kind of task in the SAP R/3 or BW system, a transaction is used. SAP provides a standard set of transactions to manipulate i.e insert, update, delete and display data in the system. But sometimes, the need to create a customer specific transaction may arise due to the following reasons

--> Standard SAP may not support that task

--> A particular transaction needs to be customized to suit the customer requirements

A transaction is defined as a sequence of dynpros (sap term for screens) having input and output fields and corresponding processing logic behind them to perform a particular task.

We call an execution of an ABAP program using a transaction code a transaction . There are dialog , report, parameter, variant, and - as of Release 6.10 - OO transactions. A transaction is started by entering the transaction code in the input field on the standard toolbar or by means of the ABAP statements CALL TRANSACTION or LEAVE TO TRANSACTION. Transaction codes can also be linked to screen elements or menu entries. Selecting such an element will start the transaction.