Transactions in SAP® ERP enable a user or an IT specialist to access a certain functionality. There are about 120 thousand of them. A transaction can be called by filling its code in a box on the top of the screen, or by double-clicking on its node in a menu.
When you log-on to SAP, then you normally enter the main menu (main screen). In a standard SAP, it corresponds to transaction S000. Each user may change the start-up transaction by entering it into his user defaults (trans. SU3), but it is usually practical only in certain industry solution (for example, SAP Retail proposes to start with W10T which is better suited to the needs of retail companies).
On the main screen, you can usually choose between showing your "user menu" (it only contains a limited range of transactions which you are authorized to) or "SAP menu" (showing all available SAP transactions organized into hierarchies) by clicking on an appropriate icon.
To display transaction codes on the main screen, go to Extras - Settings (on the horizontal menu) and then tick "Display technical names".
Due to the enormous quantity of SAP transactions, only a limited selection will be described in this handbook. If you know the code (see above how to obtain it), then you may search for it in an alphabetically sorted list under Category:Book:SAP ERP (note: the list contains all chapters of this book). Else, look for the appropriate Business scenario, IMG activity (for IT specialists) or Generally useful transactions.