The Quarians are a species of humanoid aliens. From the neck down they have the same build as a human, only their facial features separates them. It's rare to see a Quarian's facial appearance properly because they always wear a full body suit when they're away from the Migrant Fleet.
The Quarians are a nomad species who have travelled the Galaxy for 300 years in a huge group of Starships known as the Migrant Fleet. They were driven out of their home world 300 years ago, after losing a war to the Geth - an artificial species created by the Quarians.
 The Geth
The Geth were created by the Quarians for cheap labor. They were created as a Virtual Intelligence, however the Quarians continued to make small modifications here and there to enable them to do more complex tasks. The Geth are all connected by a neural network, so when the Quarians built more Geth, their intelligence expanded. One day a Geth asked his master what his purpose in life was.
 The Geth Rebellion
The Quarians realised they had created a Artificial Intelligence, which meant that they were now using the Geth as slaves. They knew the backlash this could cause and acted quickly to try and get all Geth shut down. However, the Geth realised what was going on and fought back. The war took billions of Quarian lives and drove the remaining few millions from their home planets. The Quarians left their home planets in a fleet of starships that would remain their home for the next 300 years. The Citadel Council gave them no aid or sympathy and they were removed from the Citadel Embassy and cut from all ties with the Council as punishment for bringing a new threat to the Galaxy.
 Migrant Fleet
After the Geth war, the billions of Quarians living on their home planets was cut to around 17 million who boarded the starships. For 300 years the Quarians have been living aboard 50,000 starships. The Migrant Fleet is split into hundreds of small tribes spread across multiple starships, however they still work together as one for the good of the fleet. When a Quarian reaches a certain age, they must leave the starship they were born on and find a tribe to join. They must go on a Pilgrimage as a right of passage into the new tribe. This involves leaving the Migrant Fleet and travelling the Galaxy. They can only return to the Fleet once they have found something of value to the tribe. This usually involves helpful information and spare parts that can improve their starship.