ABAP Events And Processing Blocks

Events/Processing blocks in ABAP 
Start of an event is marked by event keyword.
End of an event is indicated by start of another event.
INITIALIZATION 
Before the standard selection screen is displayed 

AT SELECTION-SCREEN
After user input on a selection screen has been processed, but while the selection screen is still active

START-OF-SELECTION
After the standard selection screen has been processed, before data is read from the logical database

GET node
After the logical database has read a data record from the node node

GET node LATE
After all of the nodes of the logical database have been processed that are hierarchically subordinate to the node in the structure of the logical database

END-OF-SELECTION
This event is generally used to mark the end of GET node event in reports where LDB’s are used

List processor events:

TOP-OF-PAGEIn list processing when a new page starts. Frequently used in Simple Reports to display header data.
This event is executed whenever the first WRITE statement is encountered. System first executes the TOP OF PAGE event and then processes the WRITE statement.

END-OF-PAGE
In list processing when a page ends. Generally used in Simple Reports to print footer data.

AT LINE-SELECTION
Triggers when a specific line in the output is selected. Generally used in Simple Drill down reports for navigation. sy-lisel holds the content of the line that was clicked

AT PFnn
When the user triggers the predefined function code

PFnnAT USER-COMMAND
When the user triggers a function code defined in the program