These statements define data types or declare data objects which are used by the other statements in a program or routine.
TYPES, DATA, TABLES, CONSTANTS, FIELD-SYMBOLS, PARAMETERS, SELECT-OPTIONS
Constants This keyword is for declaring constant identifiers.
CONSTANTS PI TYPE P DECIMALS 3 VALUE ‘3.147’.
Constants cannot be modified in the further program.
Field-Symbols These are similar to pointers in C language. Used for referring to any type of data object in a program.
FIELD-SYMBOLS: <FS1>, <FS2>.
DATA: A TYPE I, str(20) TYPE C.
ASSIGN A TO <FS1>.
WRITE <FS1>. //Value of A will be displayed
ASSIGN STR TO <FS1>.
WRITE <FS1>. //Value of STR will be displayed.
Types This keyword is used to define new user defined data types.
TYPES FLOATING TYPE P DECIMALS 6.
DATA A TYPE FLOATING.
DATA B TYPE FLOATING.
Tables This keyword is used to create a structure in RAM corresponding to a database table.
TABLES: mara, vbrk.
Type-Pools This keyword is used to attach a Data Dictionary (DDIC) type group to the current program. Type Group is a central repository object that maintains definitions of user defined data types.
TYPE-POOLS SLIS. //Used in ALV Reporting
TYPE-POOLS SSCRFIELDS. //Used for selection screen.
Courtesy: ABAP Technical Documentation