Usability tests come in several different flavors:

Functional: Your customers perform specific tasks with your product and I collect feedback on the problems they encounter.

Benchmark: I measure specific aspects of the users' experience, like time to completion and success rate, that can be used as metrics to measure later improvements.

Out-of-the-box: I focus on your customers' first time experience with your product and identify how that experience can be improved.

Competitive: I compare your product design with your competitors and we learn from your competitors' mistakes.