Aquila and Priscilla

Who are "Priscilla and Aquila" (Acts 18:18)?
Aquila and Priscilla were husband and "wife" (Acts 18:2) Christians who came from Rome to Corinth, where they met Paul. Aquila was a "Jew ... born in Pontus" (Acts 18:2), which is in Turkey today. While Priscilla could have been a gentile whom Aquila met and married in Rome or elsewhere, it is more likely that Priscilla was also a Jew and from Pontus, like Aquila. Like Paul, Aquila and Priscilla were tentmakers by trade and labored with him to provide for themselves while evangelizing in Corinth. When Paul went to Ephesus, Aquila and Priscilla accompanied him and then remained in Ephesus while Paul continued onward (see below).

Where is "Cenchrea" (Acts 18:18)?
Cenchrea was the eastern port of Corinth. Cenchrea thrived commercially during the Roman Empire but declined thereafter. Today, it is a small village of a few hundred people and called Kechries.

Why didn't Paul stay in Ephesus (Acts 18:20)?
See Apollos.

Where did Paul go from Ephesus?
He "sailed" (Acts 18:21) to "Caesarea" (Acts 18:22), the main port in Israel. From Caesarea, he went south by land to Jerusalem "and greeted the church" (Acts 18:22), and then went back north ("down" (Acts 18:22) in terms of elevation) to "Antioch" (Acts 18:22) of "Syria" (Acts 18:18), where his missionary journey had begun. After spending "some time there" (Acts 18:23), Paul started out again through "Galatia and Phrygia" (Acts 18:23).