Christmas is simply a celebration of the birth of Jesus Christ. December 25 is probably not His birthday, but simply a day chosen for festivities. Some complain that elements of Christmas are pagan in origin, suggesting that those who celebrate are worshiping wrong, or even worshiping false gods.
Christmas is often a source of contention between Christians and non-Christians. Celebrating is entirely optional, of course… but understanding the reasons for celebration is not. Cultists and skeptics often attack Christians via Christmas, so it helps to understand the issues involved.