Think of Acts as a sequel. The Gospels were the first part. Acts comes in and takes the story into the second part; taking the salvation message and moving it across the world.
Acts was also written by Luke, and some have called the two books 1 and 2 Luke! Luke was a Gentile (a non-Jew) and a Christian who traveled with the apostle Paul. An apostle is a "sent one", or messenger, of the story of salvation through Jesus.