Code coverage tests are White Box Tests run by the developers team as part of the developers feedback loop in order to discover areas of their code that are not exercised  by the Use Cases within their Test Suite.

Code coverage can be measured by several criteria:

• Function coverage - Has each function in the program been executed?
  • Note: implied by Path coverage...
• Statement coverage - Has each line of the source code been executed?
  • Note: implied by Path coverage...
  • Note: implied by Decision coverage (and therefore Path coverage)...
• Decision coverage (also known as Branch coverage) - Has each control structure (such as an if statement) evaluated both to true and false?
  • Note: implied by Path coverage...
• Condition coverage - Has each boolean sub-expression evaluated both to true and false (this does not necessarily imply decision coverage)?
• Path coverage - Has every possible route through a given part of the code been executed?
• Entry/exit coverage - Has every possible call and return of the function been executed?
  • Note: implied by Path coverage...

 Code Coverage Tools
Modern Code Coverage tools such as NCover, PartCover, etc.