This can range from confirming a bug, assigning it to the proper developer, or even resolving a bug. Developers should be self-explanatory. Managers are also Developers but also have the power to control access to projects. They are assigned ownership of a project on a per project basis. This is similar to a forum moderator.
Administrators are the people who have total control over the system.
Yes, they can even screw up the project. In principle, these accounts shouldn't be used except for administrative functions. Theses might include creating new projects, assigning managers to projects, or making system changes. User discovers what he considers a bug. User returns to the product webpage to see if there is a fix or workaround.