Since a project has a beginning and an end, it goes through what is called a life cycle.
The project life cycle has four phases. The amount of effort that each phase requires varies per project. However the sequence of the four phases is holy.
During the project initiation phase the decision is to do the project is made. It starts with someone identifying the need for a product or service. Once the decision to do the project is made, you go to the next step
The planning phase is the most important phase of any project. Well all phases are important. What I mean is that the planning phase is where you set the basis for the outcome of the rest of the project. Here is where you determine your project schedule and budget and write your project plan. Anything you forget in the planning phase, will show up in the execution phase as an annoying and often costly problem. When tempted, never give in to cutting corners in the planning phase of a project.
Once the project plan is completed and approved, it is time to get cracking and start to execute the project. This is the phase where you use performance measurement tools to manage the scope schedule and budget of the project.
The most exciting part of the closing phase is verifying that the created product or service meets the requirements set forth in the beginning of the project.
Once these requirements are met and accepted by the client, you have to close all activities such as contracts and financial reports and prepare the final documentation of the project.
Note that the project life cycle is actually a process and not a cycle.
This process also makes it clear why the sequence of the phases is holy: imagine executing a project before properly planning it.