Difference between the use of Type, Types and TYPE POOL in SAP ABAP programming.

TYPE keyword is used for Data or FIELD Symbol declaration

You use TYPES keyword for defining local types

TYPE POOL is used for grouping local types so that you can use them without duplication

TYPES: BEGIN OF z_t000,
   Name TYPE c Length 10.
   N_id TYPE i.
TYPES END OF z_t000.

One of the common examples of TYPE POOL is Type Group ABAP. Navigate to Transaction SE11 and select Type Group and type ABAP there then press F7. You will see the body of the type group which is defined by the keyword TYPE-POOL.