Tatian was a disciple of Justin Martyr, but after Justin's death he was influenced by gnosticism and started his own sect, called the Encratites.
"Encratites" means the self-controlled or the masters of themselves. It's believed that he founded the sect around A.D. 166. His death is placed in 172.
He wrote an Address to the Greeks, which is orthodox; however, he is most famous for his Diatessaron, which is a harmony of the Gospels, the earliest testimony to the text of our four Gospels. He wrote it after his influence from gnosticism, though, so he left out all the parts where the Gospels emphasize Jesus as a physical. Gnostics believe that all material things are bad, so the Christ couldn't really have been fully human.
You can read his writings, along with just about all other pre-Nicene (before A.D. 325) Christian writers, in The Ante-Nicene Fathers (10 Volume Set). If you go there, you may want to purchase the volumes separately, as the first 3 contain all the 2nd century writings.
Tatian's writings are in volume 2, though his Diatessaron, harder to come by in the late 1800's when The Ante-Nicene Fathers was published, was held until volume 9.