In order to find out the relation between a program & transaction code first we have to understand what is a transaction code. For executing an executable program we use transaction code. In simple words T.Code is nothing but a shortcut to execute a program.
Use the T.Code SE93 and enter the name of the Transaction code to find out the name of the program. Also you can use SE80 and then where used etc. To know which program assigned to transaction(for ex. SE91,or System-Status),then go SE38 and push button Display Object List.
On the contrary if you want to find out the T.Code associated with the program go to the database table TSTC. Enter the name of the Program and press execute.
The associated transaction code ( if any ) will be displayed.