The Patrick Smacchia NDepend site has a good set of definitions: Definitions of indicators . His NDepend tool (for .NET) provides many different ways to quantify a code base.
How useful the metrics are, well, is a bit more controversial. They are good for getting a code-level view of the sky, but are they useful every day? How many stores do they use (or care) on a weekly basis? I suspect that most of them are more concerned about the number of errors: the number of errors corrected and the number of errors recorded.
Of course, measures, such as linkage and communication, provide indicators of possible refactoring points.
Another important metric: how many tests exist, and about as much as the code coverage these tests represent. Refactoring without tests and good testing is a risky business.
Software indicator
Code metrics overview
source share