The following are helpful Project Server Collaboration definitions:

• Risks   Risk management involves proactively identifying and tracking events and conditions that can potentially affect future planning for a project. Risks can be rated by severity and assigned an estimate for how likely they are to occur.
• Issues   Issues can be used to track work items that are not easily identified as assignments or tasks in projects. Project managers, resource managers, and team leads can then determine whether the issues will affect the project and identify strategies to manage the issues before they become larger problems.
• Documents   A document library provides a location for team members to collaborate and share documents and other project artifacts with project team members.
• Project Site   A project site can exist for each project saved to the Project Server database. The project site enables additional collaboration features (for example, announcements, alerts, deliverables and the ability to link these to your project tasks) by providing a Web site for each project that makes the capabilities of SharePoint Foundation available to a project team.

For more information review the Microsoft TechNet article:  https://technet.microsoft.com/en-us/library/hh767484(v=office.14).aspx