DTA and DTL

It's very big concept.
Cannot be explained over here.
Better refer AS 22.
You will get your answers.

Ok...Thank you.

Then tell me,what is the main purpose of recognising DTA and DTL.

for understanding the concept of DTA / DTL, please read carefully the objective of AS 22 or IndAS 12. It is self explanatory and will give you whole idea of this standard.

DTA and DTL are created in case there is any timing difference
[for example dep as per CO. ACT is more and dep as per IT ACT is less that means in future dep as per CO ACT will be less and dep as per IT ACT will be more.]
Therefore you are more tax now (as dep is less as per IT ACT) and in future you will pay less tax (as dep will be more)
the difference between wt ur paying now and wt u will be paying in future is the DTA (as benefit is still receivable from the extra tax paid now)