No two that are ever the same. Each project contains unique attributes that must be considered and managed through strong project management methodologies and techniques.
The first unique attribute to every project is that all project tasks are performed by people. People with their own communication styles and approaches for managing project tasks and responsibilities. Melding these individuals together to create a cohesive and efficient team is a challenge every project, and Project Manager, must address.
Another unique attribute of projects is that every project is constrained by limited resources (time, staff, etc.) To handle these limitations, the project must be carefully planned, executed and controlled. A project is given a limited timeframe in which to produce the end product/service. The end result is accomplished by a team created specifically for that project. If a project is lengthy, it is not unusual for new team members to replace existing members as needed. This can cause additional challenges as new team members often bring with them different approaches to project activities that are now their responsibility.
In the end, every project is unique because it creates a unique product or service. Each product or service is the result of project tasks performed in distinct steps and at steady increments.