Your tank is the product team. They decide what the group is going to do, the pace of work, and generally what needs to be done.

Your DPS are the design team. They follow product's lead in terms of what should be done, but are pretty much on their own to decide the specifics of implementation.

Your healer is the development team. All they want to do is help things progress, but they're too busy trying to make up for the issues caused by product and design's decisions.

(And yes, I am both a healer and a software developer, I may be a bit biased )