These were born-again people, and they received the baptism of the Holy Ghost after they believed. Of course, we know from other Scriptures that every believer has the Holy Ghost, but there is a further experience as this Scripture plainly teaches. This isn't the fullness of what God has for His saints, but we must all press toward the fullness of what God desires to give to His own.

Acts 8:14 Now when the apostles which were at Jerusalem heard that Samaria had received the word of God, they sent to them Peter and John:

Acts 8:15 Who, when they were come down, prayed for them, that they might receive the Holy Ghost:

Acts 8:16 (For as yet he [the Holy Ghost] was fallen upon none of them: only they were baptized in the name of the Lord Jesus.)

Acts 8:17 Then laid they their hands on them, and they received the Holy Ghost.