Patti;
I have been going to WDW during December, usually the week before Xmas, since 1996. The weather can be very pleasant with highs in the mid-to-upper 70s and lows in the lower 60s to upper 50s. It can also be chilly. I can remember one year where the temperature reached a low in the low-to-mid 20s with the farmers worried about frost damage to the orange crop. You need to bring clothes for either condition but its mostly nice during the day and you may wish to consider a sweater or light jacket for evening activitities. I enjoy Disney during the Xmas season; I believe that you will as well. I don't recall a lot of rain during December but there would be occasional days (Xmas last year it rained - first time I went during Xmas to New Years Eve week) when it rained.
The first time that I went to WDW was in July 1995 as the recommendation of a friend. I felt like I walked into a shopping center with the roll of stores. It was hot, temperatures in the upper 90s to lower 100s, and crowded and had to wait to get on rides. It was not an enjoyable vacation. I couldn't figure out why this was considered fun! My friend recommended going back during a different time of the year and so I went back a year later at Xmas. I enjoyed it and have been going back every year since (it helps that I really like Xmas and Disney does Xmas well). The temperatures for the most part are pleasant and as noted, bringing a jacket or sweater for evening activities such as Illuminations may be recommended.