Apollos was a Jewish teacher from Alexandria, described in Acts 18 as "an eloquent man, and mighty in the scriptures," who taught in Ephesus and later in Corinth.
Acts 18:24-25 in the King James Version reads:
24 And a certain Jew named Apollos, born at Alexandria, an eloquent man, and mighty in the scriptures, came to Ephesus. 25 This man was instructed in the way of the Lord; and being fervent in the spirit, he spake and taught diligently the things of the Lord, knowing only the baptism of John.
These verses introduce Apollos near the close of Acts 18. Read it in context at Acts 18:24-25, or the full chapter at Acts 18. Encyclopedia entry: en.wikipedia.org/wiki/Apollos.