Test coverage is nothing but covering of the all the 
requirements to writing the test cases

Test coverage is having the spreadsheet which tell u that
whether all the modules & submodules get get covered in our
test cases or not. from these spreadsheet one can easily
understand the scope of our test cases.
So the different columns in the test coverage may be as -
Module, sub module, Test case ID, test case name, tested or
not tested, (if tested then) no. of defects, type of
defects, status, etc.......

Sameer

coverage of test case should be good,that means each 
requirement should be covered with one testcase