Similar to services, while the action is being performed an action server sends progress feedback to the client.

Actions are not useful when a response may take a significant length of time. 

Actions allow a client to cancel the request before it completes..

Similar to services, action clients send a request to an action server in order to achieve some goal and will get a result.

nav2_map_server / map_server

nav2_waypoint_follower / waypoint_follower

nav2_planner / planner_server

nav2_bt_navigator / bt_navigator

nav2_amcl / amcl

nav2_controller / controller_server

nav2_lifecycle_manager / lifecycle_manager

A BT is a way to develop a single task in an autonomous agent.

BT are very popular in computer game programming, AI and Robotics.

BTs are a very efficient way of creating complex systems that are both modular and reactive.