The flow of triggering is like this
Initialization
at selection-screen
start-of-selection

Tables can be created with top-down approach i.e. first create data element , then domain , then field. Bottom down approach is first create field ,dataelement and domain.

We can Validate Selection Screen With the Help of the Following Events, the Event Follows the Same hierachy.
AT SELECTION-SCREEN ON
AT SELECTION-SCREEN ON BLOCK
AT SELECTION-SCREEN OUTPUT
AT SELECTION-SCREEN

By using upload command userinteraction is must for every record,where as in WS_UPLOAD user interaction is not needed.

SAP memory is a memory area to which all main sessions within a SAPgui have access. You can use SAP memory either to pass data from one program to another within a session, or to pass data from one session to another. Application programs that use SAP memory must do so using SPA/GPA parameters .

ABAP memory is a memory area that all ABAP programs within the same internal session can access using the EXPORT and IMPORT statements. Data within this area remains intact during a whole sequence of program calls. To pass data to a program which you are calling, the data needs to be placed in ABAP memory before the call is made. The internal session of the called program then replaces that of the calling program. The program called can then read from the ABAP memory.

Logical Database is an ABAP pgm which retrieves data from various tables which are interrelated and provides a readolny view of the data.

Function modules is like a small program (is not a suboutine because sub-routines are not standalone objects-i.e we cannot execute them alone) which are similar to 'functions' in C.
we can pass the varible and get the return values even with out knowing what is happening in its code.
they are standalone programs-i.e.we can debug and execute them alone unlike subroutines and macros and includes.