In project's workspace as provided by Udacity (VM) required python package rospkg is missing. It is mandatory to run pip install rospkg prior to any test or turtlebot world won't run in gazebo.
add_markers initial behavior require to display the marker at pickup, hide it for 5 seconds then display it again at drop point. This behavior doesn't exist anymore in the submited version since this node is integrated with pick_objects.
Localization is implemented here using AMCL ros package: http://wiki.ros.org/amcl
The algorithm here is Adaptive Monte Carlo.
Mapping is done with gmapping ros package using laser sensor and robot pose: http://wiki.ros.org/gmapping
In a previous step we drive the robot into the environment to generate a map file (pgm) that will be used afterwards for localization and navigation.
Navigation is done with move_base ros package: http://wiki.ros.org/move_base
This package is able to compute a path using the map and drive the robot to a goal. Also there are messages allowing to monitor the state of the robot (position and status reaching the goal)