Arius of Alexandria (c. 250-336) was not the original inventor of the heresy that bears his name, but he was by far its most famous and influential advocate. Nowadays, "Arianism" is often used as shorthand for any theology that maintains that Jesus was not divine. However, during the third and fourth centuries, there were a number of competing Arian and semi-Arian positions; the position that Arius himself endorsed was not the most radical of them.
Little is known about Arius' early life. He was probably born in Libya. According to Epiphanius -- a fourth-century heresiologist who spends quite a lot of time ranting about Arianism in his book Panarion ("The Medicine-Chest") -- Arius was tall, handsome, popular, and well-spoken. In 313, Arius was ordained as a presbyter in Beucalis, a region of Alexandria, and spent much of the next decade arguing with local bishops about the nature of Jesus' sonship.
The arguments made Arius some powerful enemies. In 321, he was excommunicated and exiled by the bishop Alexander. He travelled to Palestine in search of allies; he found a staunch one in Eusebius of Nicomedia and a more moderate one in Eusebius of Caesarea. A reconciliation between Arius and Alexander was attempted, but it went poorly. Eventually Arius suffered defeat at the Council of Nicaea and his position was condemned in favour of a Trinitarian theology.
This time Arius was banished to Illyria, but once again his smooth talking served him well, because he made friends in the court of the Emperor Constantine and was formally welcomed back into the church and his home city of Alexandria in 331. (In an odd twist of fate, his orthodox opponent, Athanasius, was exiled for a while.)
Arius, or so the story goes, died rather gruesomely the day before the ritual of reconciliation was to take place. The Nicene party claimed that God had made his opinion of Arius clear by means of this violent refusal to let him take Communion. The Arians, for their part, suspected that their leader had been poisoned.
Arius believed that Jesus was the eldest and brightest of all creatures, but that he was still a creature for all that. In other words, Arius' Jesus had a beginning -- or, as his party's slogan put it, "There was a time when he was not!" In 325, the Council of Nicaea condemned this view, asserting that Jesus is divine and coeternal with God his Father, and anathematizing anyone who disagreed.
It's worth noting that Arius believed that Jesus was created before everything else in the universe. He did not think Jesus was a "mere" human being who just happened to have God's blessing along with some magical powers. (That view is better characterized as adoptionism, and it predates Arius by half a century.)
In the decades following Arius' death, theologians like Aetius and Eunomius would go further than Arius ever did, claiming that Jesus was so different (anomoios) from the Father that he was even capable of sin. This view is variously called Aetian, Eunomian, or Anomoian Arianism, but it never became popular.
Further Reading:
Robert Gregg and Dennis Groh,
Early Arianism: A View of Salvation (Fortress, 1981)
Rowan Williams,
Arius: Heresy and Tradition (Darton, Longman, and Todd, 1987)
http://www.earlychurch.org.uk/arianism.html