But there was a certain man, called Simon, which beforetime in the same city used sorcery, and bewitched the people of Samaria, giving out that himself was some great one:
Note 1 at Ac 8:9: The only biblical information we have about this Simon is in this account in Ac 8. From this we can see he used sorcery to convince the people that he was "the great power of God" (Ac 8:10). The entire city believed him until Philip came along with the true power of God, preaching the Gospel of Jesus.
Ac 8:13 says that Simon also believed and was baptized, but it is questionable whether it was a true conversion because of Peter's statement in Ac 8:23 that Simon was "in the gall of bitterness, and in the bond of iniquity." Peter said this to Simon because Simon had offered them money if they would give him the power that they had to lay hands on people and see them receive the Holy Ghost.
It appeared that he still wanted to portray himself as someone special to the people, and he had just switched from using sorcery to using the Gospel to do it. It is to Simon's credit, though, that when rebuked by Peter, he didn't appear to rebel but prayed that none of those things would be so.
There are a number of extra-biblical writings about Simon. They say that he claimed to be the Messiah, the Son of God. It is also written that Simon returned to sorcery after the happenings of this chapter and became a constant antagonist to the Apostle Peter. While interesting, we cannot base any conclusions on these writings outside of Scripture.