Pergamos was a city in western Asia Minor (modern Turkey), north of Smyrna, and about 15 miles inland from the Aegean Sea. For a while it was the capitol of the Roman province of Asia. Pergamos was famous for its library and beautiful architecture. It also contained a complex of temples to Zeus, Athena and other Greek gods, and also temples to some Roman emperors. The church of Pergamos is the 3rd of the 7 letters written in Revelation, chapters 2 and 3.
The letter calls Pergamos "Satan's Throne" (this was probably in reference to emperor worship and other pagan practices that flourished there), and accuses some people there who hold to the teaching of the Nicolaitans (the Nicolaitans were people who blended Christianity and paganism). The letter then calls for the people to repent, it also admires the church for not denying its faith in Christ, in face of the martyred faithful witness, Antipas.
The other six churches that are mentioned in this part of the book of Revelation are Ephesus, Laodicea, Philadelphia, Sardis, Smyrna, and Thyatira.