Hi Mallika,
Forward and Backward traceability is known as Bidirectional.
Forward traceability is from requirements to design to code to testcases.
Backward traceability is on the reverse direction , that the end product has met the requirements or not. It is very difficult to do this traceability without tool .
you can surf few links on it, like :-
http://www.sdmagazine.com/documents/s=759/sdm9904d/9904d.htm have nice time,
Ramesh