Table deceleration

1. Types: begin of itab ,

…..

…..// Structure creation

End of itab.

Data: gt_itab type table of itab, // internal table

Gw_itab type itab. // work area

2. Data: begin of itab occurs 0 with header line.

….

End of itab.

Question:

1.What is the difference between this two type (memory allocation?)

2.Why prefer the first method to declare the internal table, why we r not giving priority for the second type?

3.Why we r creating separate work area in the first case. And how come it differs from the second case?

Please give the detail explanation…