Christmas is celebrated to remember the birth of Jesus Christ, who Christians believe is the Son of God.
The name 'Christmas' comes from the Mass of Christ (or Jesus). A Mass service (which is sometimes called Communion or Eucharist) is where Christians remember that Jesus died for us and then came back to life. The 'Christ-Mass' service was the only one that was allowed to take place after sunset (and before sunrise the next day), so people had it at Midnight! So we get the name Christ-Mass, shortened to Christmas.
Christmas is now celebrated by people around the world, whether they are Christians or not. It's a time when family and friends come together and remember the good things they have. People, and especially children, also like Christmas as it's a time when you give and receive presents!