what are the different types of ros_control ?

edit

To answer questions, 1 & 2. The reason they have two slightly confusing descriptions in the names is that they are describing both the type of output from the controller to the joint as well as the input. For example the effort_controller/joint_position_controller receives position requests from ROS and outputs the efforts required to reach that position.

3) Whichever node is controlling your joints will be publishing a joint_state topic. These messages (may) include the efforts at each joint, this is an optional parameter so will depend on the node that you use.

4) Joint trajectories are specified as a sequence of positions, so you'll need a controller than accepts these. The output will depend on how you're controlling the actual robot, probably an effort controller.

Hope this helps.

Please see #q292248 which should clear up at least the first 2 questions you post.

As for the other two:

while using position controllers can I read the effort at each joint ?

yes, joint state is decoupled from the controller that is being used.

I'm using MoveIt! for a gazebo simulation. I've setup an action server FollowJointTrajectory. which controller should I be using for this ?

I don't understand your question. MoveIt expects a FollowJointTrajectory action server. How exactly did you "setup" one?

ros_control provides the joint_trajectory_controller which also offers a FollowJointTrajectory action server. That is typically used in setups of ros_control that need to be compatible with MoveIt.