An alternative name for the book is Acts of the Holy Spirit. It describes many of the journeys and actions taken by the apostles, meaning "those who have been sent" by God, to be His witnesses.
This was originally applied exclusively to those who had personally seen and/or lived with Jesus of Nazareth. The book of Acts contains many descriptions of miraculous events (which were given as signs from God to validate the apostles' teachings), which were performed by the Holy Spirit through the apostles. These included miraculous healings, casting out evil spirits, the raising of the dead, and also historical descriptions of everyday life in The Roman Empire and in ancient Jerusalem.