What is Kanban in Scrum?

In Kanban there is Kanban board (not a Scrum board) that has three columns: to do, doing (or WiP column) and done. For software development projects the Kanban Board can have four columns: to do, doing (assigned to a WiP column), testing (assigned to a WiP column) and done. The WiP (Work-in-Progress)is the number of tasks which the Kanban Team can face in maximum. Tasks are only pulled to the WiP and its columns when there are enough resources. Personal structures of the company can be reused not like in Scrum in which you have three roles: Scrum Master, Product Owner and Developer.