The Terran species was forced to abandon their homeworld and seek out new worlds to support their people after discovering the approach of several planet-killer classed asteroids heading towards their home planet, Terra. This exodus is detailed in the account of the Wayfarer's Collective. They seem to have a mutual loyalty to Thanerians.
According to Novus Ordo, the All-Seeing Eye to the Imperium, they of all races possibly be the most dangerous threat the Imperium faces, though some of their greatest weaknesses would be their trust and hope.
After having to flee their doomed home-planet, Terra, the Terrans traveled on fleets of generation ships, two fleets of which were known as The Cetus and The Aquarius, there were twelve fleets total. The Cetus and The Aquarius were the only two fleets known to have survived the first excursions. The Cetus later, however, succumbed to the influence of The Void, and its Terran inhabitants were never heard from again. The Aquarius made it to its new planet, which was named Aquarius Eleven, in honor of the Aquarius fleet and the Eleven fleets that were lost on their way to the new world. Aquarius Eleven is located in the Comis Quadrant.
It can be surmised that Terran culture was similar to that of modern day human culture. Their original planet, Terra, consisted of multiple nations, each with its own unique culture. Their lifespans are similar to that of modern-day humans as well.