Sugar plums were dried fruits coated with sugar. Plum pudding is a caky pudding that contains quantities of dried fruit -- making it a suspiciously close relative of the much slandered fruit cake. Here's how it was made 100 years ago or so.
1 1/2 lb. of raisins (aka "dates")
1/2 lb. of currants (aka "raisins")
1/2 lb. of mixed peel
3/4 lb. bread crumbs
3/4 lb. suet
1 wineglassful brandy
Stone and cut the raisins in halves, but do not chop them; wash, pick, and dry the currants, and mince the suet finely; cut the candied peel into thin slices, and grate down the bread into fine crumbs. When all these dry ingredients are prepared, mix them well together; then moisten the mixture with the eggs, which should be well beaten, and the brandy; stir well, that everything may be very thoroughly blended, and press the pudding into a buttered mould; tie it down tightly with a floured cloth, and boil for 5 or 6 hours. It may be boiled in a cloth without a mould, and will require the same time allowed for cooking.
As Christmas puddings are usually made a few days before they are required for table, when the pudding is taken out of the pot, hang it up immediately and put a plate or saucer underneath it to catch the water that may drain from it.
The day it is to be eaten, plunge it into boiling water, and keep it boiling for at least two hours; then turn it out of the mould, and serve with brandy-sauce. On Christmas Day a sprig of holly is usually placed in the middle of the pudding, and about a wineglassful of brandy poured round it, which, at the moment of serving, is lighted, and the pudding thus brought to table encircled in flame.
Serves 7 or 8 people.
--from Sarah Hollis' The Country Diary Christmas Book (Henry Holt, 1993). Her source is Mrs. Beeton's Book of Household Management.