Acts is the fifth book of the New Testament and the forty-fourth book of Bible. Acts chronicles the events that took place after Jesus' resurrection, starting with His ascension into heaven and the outpouring of the Holy Spirit on His disciples, resulting in the beginning of a mighty work of God in building the church. Many such as Peter and Paul are sent by God and begin starting many churches throughout Israel and all over the world. The Book of Acts is written by Luke the Evangelist.
The Book of Acts was written by Luke as a book meant to directly follow his previous account of Jesus's life. As in the Book of Luke, Acts was written specifically to Theophilus, perhaps as to provide witness  to Jesus Christ.
Acts is written as an overview of the early churches and missions of the Apostles (the book is often called "the acts of the apostles"). The beginning of Acts also addresses the command  for the Apostle's to wait into Jerusalem to be sent out by the Holy Spirit.