ROS Indigo (LTS) on Ubuntu 14.04 (LTS)

Basically, follow the instructions below.

However, the following step is prone to failure due to keyservers being down, or having invalid certificate, etc.

sudo apt-key adv --keyserver hkp://ha.pool.sks-keyservers.net --recv-key 0xB01FA116

To solve this, go here and search for 0xB01FA116 key.

Copy the key text and save it in a text file.

vim ros_key.txt
# paste the key text copied from the site above.

sudo apt-key add ros_key.txt

Continue with the rest of the installation.

sudo apt-get update
...

Reference