I had difficulty searching for this answer on google and debian documentation!
The FAQ page wasnt very clear either. I mean it wasnt 100% idiot-proof clear.
"testing" repo makes new updated packages available to the user (ie
Caution: testing packages may not be the most stable packages, but it is more stable than the "unstable" repo.
Note the format of the repo line added.
Note in the above line "wheezy" repo is debian 7.0 release default repo. Unstable repo can have error prone packages.