Project Control Variables
Project Management tries to gain control over five variables:

Time - The amount of time required to complete the project. Typically broken down for analytical purposes into the time required to complete the components of the project, which is then further broken down into the time required to complete each task contributing to the completion of each component.

Cost - Calculated from the time variable. Cost to develop an internal project is time multiplied by the cost of the team members involved. When hiring an independent consultant for a project, cost will typically be determined by the consultant or firm's hourly rate multiplied by an estimated time to complete.

Quality- The amount of time put into individual tasks determines the overall quality of the project. Some tasks may require a given amount of time to complete adequately, but given more time could be completed exceptionally. Over the course of a large project, quality can have a significant impact on time and cost (or vice versa).

Scope- Requirements specified for the end result. The overall definition of what the project is supposed to accomplish, and a specific description of what the end result should be or accomplish.

Risk- Potential points of failure. Most risks or potential failures can be overcome or resolved, given enough time and resources. According to some definitions risk can also be negative, meaning that there is an opportunity to e.g. complete the project faster than expected.

Three of these variables can be given by external or internal customers. The value(s) of the remaining variable(s) is/are then set by project management, ideally based on solid estimation techniques. The final values have to be agreed upon in a negotiation process between project management and the customer. Usually, the values in terms of time, cost, quality and scope are contracted.

Reference: http://en.wikipedia.org/wiki/Project_management#Project_Management_activities