In reply to burra thanuja:
The word "checker" has several meanings depending on context and local termonology, and it's even a SystemVerilog keyword for a specific assertion based construct.
But generally, a checker is a passive component or procedure that compares expected behavior with actual results.
A testcase could be considered a checker in itself, or a collection of checkers. Usually a testcase configures the checkers we want to use and provides the stimulus as well.