What does source command actually do?

edit

Hi, I've read that $ source devel/setup.bash is the same as $ .devel/setup.bash and that it reads and executes setup.bash script in the current shell. Opposite to this $ ./devel/setup.bash what would run the same script but launching new shell.

So we want to run $ source <filename> command everytime we open new terminal.

Following questions are:

1. Its not clear to me what actually am I doing with the source command. Where can I find this bash file that contains these enviroment variables, so I can track what's going on?
2. Can I permanently add required enviromental variables so I don't have to do it manually every time I open a terminal? What are pros and cons of doing such.
3. I followed tutorial. Entered this:

$ source devel/setup.bash

$ echo $ROS_PACKAGE_PATH

Which gave me:

/home/username/Workspace/ros-workspace/tutorial_ws/src:/opt/ros/indigo/share:/opt/ros/indigo/stacks

Then I entered this:

$ source /opt/ros/indigo/setup.bash

$ echo $ROS_PACKAGE_PATH

Which gave me:

/opt/ros/indigo/share:/opt/ros/indigo/stacks

Why such sequence erased my ~/home... path from bash??