Traceability Matrix: 

It's a document that maps requirements to Testcases.
For every requirement there will be  a Requirement ID, Req.
Description. For each Requirement there will be one more
testcases. In the same way for every TestCase there will be
a TestCase ID, Testcase Description.

USE: 

Using Traceability Matrix we can tell the coverage of the
Testcases.

i.e WE can identify the requirements which have bug fixes
and related functionalities.

Trecebility matrix is mapping between software Requierment
and Test case . It is use to make sure that test case
consists all Functional  Requierment .

Tracability matrix is the mapping between test cases and 
requirements.

By using treasability matrix, we can ensure, that we cover 
all the requirements in testing.

we can use treasability matraix in testing

Treasability Matrix is one of the document use to maps the 
requirement with the Test cases done by tester.
Tester prepare this doc after the Test cases designed.
It is use in the testing processes in CMM company to ebsure 
that i covered all the req in designed test cases.We have 
to prepare this in Excel sheet only like following 
content...
Main content in it  
Unique identification no,Req statement,soource of req,srs 
requirement paragraph no where requirement mention,Test 
design paragraph no where requirement mention,Test,test 
specificetion paragraph no where requirement 
mention,Test,program sourece paragraph no where requirement 
mention,Test,test case no,test req no,modification if 
req,comment /remark