Rather than give my full writing, I thought I might more put some notes here.
1. In Acts 8, at least the Hellenistic Jewish believers are scattered. They seem to be the primary target of Paul's (Saul's) persecution. Either they were causing more trouble than Peter and the original disciples or their community had a bigger issue with them than Jerusalem had with Peter.
2. No matter. God uses it for good. If the boundary between Aramaic and Greek-speaking Jews is crossed in Acts 6, the boundary between Judea and Samaria is crossed in Acts 8. This is the second part of the prediction in Acts 1:8. First they would witness the resurrection in Jerusalem (Acts 1-7). Then they would witness in Judea and Samaria (Acts 8-12).
Samaria was a different culture than Judea, and the people of each didn't really get along well with each other. Samaria's understanding of Israel's faith was a little different too. The word Jew comes from Judea. It started to be used after some descendants of those who went as captives into Babylon and Persia returned to Judea. Of course many remained in those far off lands too.
The Samaritans thus did not consider themselves Jews, even though they traced their ancestry back to the northern kingdom. They had their own distinct form of Israelite faith. They were more syncretistic. They had their own Pentateuch. For a long time they had their own temple.
3. The incident with Simon the sorcerer is interesting. Later Christian tradition would see him for some reason as the beginning of all heresy. He must surely have been someone of renown, someone Luke wanted his audience to know had been out-miracled and outclassed by Philip.
He was not quite a believer, the archetype of the stereotypical televangelist who is more interested in putting on a show and making money than truly following Christ.
Some are baptized in Samaria, but there is a problem. They have not received the Holy Spirit. Baptism means nothing if it is not authenticated by receiving the Spirit because it is the Spirit that makes one truly a Christian. The apostles Peter and John come up to lay hands on them so that they will receive the Holy Spirit and truly become part of the people of God. Luke may also want us to take away from this incident that Philip's ministry was subordinate to that of the apostles.
4. The content of Philip's preaching was the "good news of the kingdom of God" (8:12). Notice that the gospel here is the coming of the rule of God. Philip also preached the name of Jesus Christ, the one to be king on earth in that kingdom.
An angel of the Lord directs Philip on a path where he runs into a eunuch from Ethiopia. This is another boundary crossing, since this person was seriously and permanently unclean on the basis of the OT. This is part of God doing away with the purity boundaries of the OT. Kings sometimes made those who watched over their harem eunuchs so that they had a guy in charge, but they didn't have a guy in charge, if you know what I mean.
Baptism was a part of the message. When Philip explains the spiritual interpretation of Isaiah 53, the eunuch seeks out water. We should assume he also received the Holy Spirit.
5. You might notice that Acts 8:37 is missing. It is not in the vast majority of manuscripts. Some well intentioned copyist must have added it thinking that Philip would have made some sort of confession of faith at this point. It is possible that Acts was used in the second century as a kind of blueprint for how the church was supposed to operate. As such, someone arguably created a version of Acts with a lot of extra details.