Carols were first sung in Europe thousands of years ago, but these were not Christmas Carols. They were pagan songs, sung at the Winter Solstice celebrations as people danced round stone circles. The Winter Solstice is the shortest day of the year, usually taking place around 22nd December. The word Carol actually means dance or a song of praise and joy! Carols used to be written and sung during all four seasons, but only the tradition of singing them at Christmas has really survived.
Early Christians took over the pagan solstice celebrations for Christmas and gave people Christian songs to sing instead of pagan ones. In 129, a Roman Bishop said that a song called "Angel's Hymn" should be sung at a Christmas service in Rome. Another famous early Christmas Hymn was written in 760, by Comas of Jerusalem, for the Greek Orthodox Church. Soon after this many composers all over Europe started to write 'Christmas carols'. However, not many people liked them as they were all written and sung in Latin, a language that the normal people couldn't understand. By the time of the Middles Ages (the 1200s), most people had lost interest in celebrating Christmas altogether.
When Oliver Cromwell and the Puritans came to power in England in 1647, the celebration of Christmas and singing carols was stopped. However, the carols survived as people still sang them in secret. Carols remained mainly unsung until Victorian times, when two men called William Sandys and Davis Gilbert collected lots of old Christmas music from villages in England.
Before carol singing in public became popular, there were sometimes official carol singers called 'Waits'. These were bands of people led by important local leaders (such as council leaders) who had the only power in the towns and villages to take money from the public (if others did this, they were sometimes charged as beggars!). They were called 'Waits' because they only sang on Christmas Eve (This was sometimes known as 'watchnight' or 'waitnight' because of the shepherds were watching their sheep when the angels appeared to them.), when the Christmas celebrations began.