What is the difference between project manager and line manager?

A Project Manager manages the work taken up by a single project whereas the Line Manager will be managing the work taken up by a line of projects.

Usually projects in organizations are aligned based on the line of business, catered to, by the project. Hence, they will have a Line Manager who manages all those projects

The Line manager will interact/liase with the Project Managers who manage the projects that fall in his line.