Test metrics is a mechanism to know the effectiveness of 
the testing that can be measured quantitatively. It is a 
feeback mechanism to improve the testing process.

They are collected at each phase of the etsting lifecycle.

Adv : 1. Better knowledge of the project.
2. Identify risk areas that requires more testing.
3. Measure of effectiveness and efficiency of testing.

5 Testing metrics :

1. Defect by action taked :
This metrics is an indicator of the effectiveness of the 
testing process and test execution . Good percentage of 
defects marked as "Rejected" implies that the understanding 
of the test engineer on the functionality is low.

2. Defect by injection phase :
Indicator of the most problematic phase in the s/w dev.
Like Design, coding, Documentation, Test planning etc..

3. Defects by Detection phase.
Indicator of the most efficient phase of s/w dev. in terms 
of finding and removing bugs.

Hope u hv an idea now.... :)
Some other test metrics 
4. Defects by priority
5. Defects by severity
6. Time to find a defect
7. Time to resolve a defect
8. Test case effectiveness
9. Defect / KLOC
10.Bad fix ratio..
and there are a lot more.. :)

Cheerz,
Saurabh Garg

Measuring of the testing activity is called as testing
metrics.Some of the fundamendal metrics include
time,cost,number of product failure,bugs found,Test
effectiveness and test efficiency.