The Decision-maker
The person (or "body" of people) who make the final decision. The decision-maker controls the "purse strings" and has veto power. The decision maker in any given organization can change depending on the scope and type of service, but there is usually only one decision maker for a given opportunity. The decision maker for an audit might be the owner of the company but the decision maker for a $2,500 special project might be the controller.

The User(s)
The person or people who will be using whatever services are provided. The user works most closely with the professional(s) who provide the service. The user generally plays a significant role in the decision making process. There can be one or multiple users depending on the particular opportunity. For audit services, the user might be the CFO but for a software consulting project, the users might include all those who use the software.

The Influencer(s)
The people inside or outside the organization who usually do not "get a vote" in the decision-making process but can significantly affect its outcome. The influencers might be someone like a bank loan officer, an accountant if legal services are the issue or an attorney if accounting services are involved. Even a secretary or family member without an official status in the buying process can have some influence in the buying process.

The Coach
A person inside or outside the organization that can provide the professional with useful "insider" information-e.g. company politics, culture, terminology, etc. The coach wants to see the professional succeed and therefore is willing to help in the sales process. The coach could be a decision-maker, user influencer, or even someone who has less power in the sales process such a secretary or book keeper. If you don't have a coach, you need to create one!