Heuristic evaluation is done by looking at the interface and trying to find out what is bad and what is good about this interface. The goal of heuristic evaluation is to find usability problems. The more people evaluators the better.
The single evaluator will miss most of the usability problems in an interface. Averaged over six projects, single evaluator found only 35% of the usability problems in the interface. It is recommended to use about five evaluators, and certainly at least three.
The differences between heuristic evaluation sessions and traditional user testing are that observer answers questions from the evaluators during the session. For traditional user testing, one wants to discover the mistakes users make when using the interface. For heuristic evaluation it would be unreasonable to refuse to answer the evaluators’ questions.