The custom of sending Christmas cards started in Britain in 1840 when the first 'Penny Post' public postal deliveries began. (Helped by the new railway system, the public postal service was the 19th century's communication revolution, just as email is for us today.) As printing methods improved, Christmas cards were produced in large numbers from about 1860. They became even more popular in Britain when a card could be posted in an unsealed envelope for one half-penny - half the price of an ordinary letter.
Traditionally, Christmas cards showed religious pictures - Mary, Joseph and baby Jesus, or other parts of the Christmas story. Today, pictures are often jokes, winter pictures, Father Christmas, or romantic scenes of life in past times.