There are two versions of Plum Pudding. Both are steamed puddings. The first is the one that you will expect, that is associated with Christmas and doesn't contain plums -- or even dried plums (prunes.)
There are many variations on it. Some people insist suet must be involved. Some say they find suet makes the pudding greasy. Many report that using vegetarian suet makes no difference in taste. Other variations include alcohol, or are alcohol free; modern versions are even designed to meet special-needs diets such as low-cholesterol diets, etc. And there's nothing wrong with variations for special diets -- depending on who is at your table, who amongst us could dispute that there may be a time and a place for nut free recipes, for example.
Plum Pudding is usually made in advance, left to mature like a fruit cake, and then resteamed for about two hours on Christmas day before serving. A microwave will do this reheating in a matter of minutes, though some people who aren't normally fussy point out that you're depriving the pudding of those two additional hours of steam in which the flavour marries and develops, and they may have a point. But, on Christmas day, you might well wonder how on earth these people manage to have a burner free on their stoves that they can sacrifice for two hours before the big meal of the year. Most of us are juggling and swapping pots and burners like mad as it is with 4 burners, and if you reduced us to only 3, we'd be a candidate for kitchen hari-kari.
Some like to age their Plum Pudding for a year in advance, some say only age it for a few months. (The sugar and the alcohol act as a preservative.) The minimum seems to be 14 days. Many people poke holes in it during the maturing process, and drizzle very small amounts of additional booze in the holes from time to time. Don't booze it too much, though, or it may be so moist that it won't stay together when you turn it out onto a plate for serving, and if you plan to light it, you may end up with more of a fricassé than a flambé.
The second version of Plum Pudding actually is a Plum Pudding that contains plums. The plums are encased in a pie shell in the pudding dish and then steamed. It is made at any time of the year and is not particularly a holiday dish.
Its earliest known ancestor comes from the 1400s, though it was as like the Christmas Plum Pudding we know today as we are like our amoeba ancestors. It contained meat and root vegetables in what was essentially a frumenty mixture that was boiled in animal stomach lining. At first, it was meant to be a meal on its own, in the lead-up to the fasting at Christmas, when the Church made people fast instead of feast at Christmas. Later, it was served at the start of the Christmas meal. Dried fruit such as raisins and prunes were introduced in the 1500s as they became accessible and more affordable to people (remember, Medieval cooking was a lot like Middle Eastern cooking today, in that it mixed sweet and savoury, fruits and meats, in the same dishes.) The meat was phased out over time, as it was in mincemeat pies.
The recipe kept evolving. In 1662, Samuel Pepys records in his diary that he ate plum porridge on Christmas Day. The animal lining was replaced by a pudding cloth, which in turn was replaced by the pudding basin, and with the advent of the pudding basin, it was no longer boiled, but steamed. Alcohol was introduced, but one remnant of its meat origins -- suet -- remained. By the Victorian era, the Christmas Plum Pudding had become such a cherished tradition that it stopped evolving -- though who knows what changes the next few hundred years hold?
Literature & Lore
To a great number of people, Plum Pudding is synonymous with Christmas pudding. If you have a pudding at Christmas, it is a Plum Pudding. But there is also a significant number of people to whom a Christmas pudding means figgy pudding or carrot pudding.