What is the difference between different controllers provided in ros_control?

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As mentioned in the official documentation, the list of available controllers are:

• effort_controllers
  • joint_effort_controller
  • joint_position_controller
  • joint_velocity_controller
• joint_state_controller
  • joint_state_controller
• position_controllers
  • joint_position_controller
• velocity_controllers
  • joint_velocity_controller

I want to know the difference between effort_controllers/joint_position_controller and position_controllers/joint_position_controller and how to use a position controller and which of the above to use?

My application is a joint velocity control of a two link manipulator in Gazebo using ROS Control.

I'm using ROS Kinetic with Ubuntu 16.04 LTS. Gazebo version is 7.12