A winter festival around the time of the solstice associated with the end of the old year and the start of the new as the sun was reborn.
A characteristic of Yule was the Yule Log which was a large freshly cut log burnt on the hearth over the Yule period. As ancient festivals tended to involve fire, the log is probably related but how is now unclear. Sometimes it was lit from the remains of old log as it was never supposed to be allowed to burn completely through. The type of wood varies from area to area. Keeping a piece in the house was thought to protect the dwelling from lightening.
Now it is regarded as a synonym for Christmas.