D&D Miniatures is a miniatures wargame put out by Wizards of the Coast that is set in their Dungeons and Dragons universe, although not in any one particular world. The game is sold in little box sets that each contain a variety of miniatures. Several different starter sets have also been made and those starter sets also contain the rules to the game itself. The hardcover Dungeons and Dragons book entitled "Miniatures Handbook" also contains the complete miniatures rules, although those rules are now out of date, as they have changed somewhat since the release of that book.
Wizards of the Coast has always been big on selling their products in ways that cause people to spend a whole lot of money on them, and the D&D Miniatures game is a prime example of this. The figures (which are plastic), come in sets of eight, and have been priced in the $9-$13 range. There have also been a few sets that contained 8 normal figures and one very large one, and those sets were priced at $20 per set.
They want to make you buy a lot of these things, so each box of miniatures contains four "Common" figures, three "Uncommon" figures and one "Rare" figure. Most of the sets of figures have contained 24 different "Rare" figures, so that means you are buying at least 24 boxes of these figures just to complete a single set of them, and there have already been 12 sets of them. That means thousands of dollars spent on what amounts to a few pounds of plastic.
These miniatures are also very popular for use with the Dungeons and Dragons roleplaying game. In fact most of the people that purchase them purchase them for exactly that reason. This is rather unfortunate because the contents of the various sets are absolutely horrible for playing Dungeons and Dragons. Using miniatures with Dungeons and Dragons essentially means that you need a dozen or two dozen figures representing men, dwarves, elves, halflings and such, and then about a million figures representing monsters. The D&D Miniatures boxes are very, very, very heavily stacked towards men, dwarves, elves, and humanoids but with very few actual monsters, and what monsters are in the sets are often oddball monsters that get almost no use in play. If you buy a lot of boxes of these figures you will end up with vast hoardes of useless men, dwarves, elves, a smaller selection of kobolds, goblins, and orcs (many of which will be in identical poses) and you won't even begin to have a decent selection of actual monster figures. Your collection might have seventy dwarves in it, but you would be lucky to have a single rust monster, and seventy dwarves are neither useful for Dungeons and Dragons or D&D Miniatures because neither set of rules properly scales to that many figures on the table.
If you simply play Dungeons and Dragons as a player, then a few boxes of these figures can keep you in character figures for a long time (and you can usually trade away the few monsters and bad guys in your boxes to your friends for more good guy figures). But if you are a game master wanting to use miniatures then you will find that you quickly end up with a very expensive miniatures collection that is so overwhelmingly full of humanoids, and you will still be missing most of the classic monsters and likely won't be able to properly field many encounters. If you are a gamemaster then I suggest actually going on eBay and spending a few bucks buying the common and uncommon figures from the recent sets (you can usually purchase all the common and uncommon figures from any given set in a single auction from many sellers, for almost no money), as those will give you all the bulk humanoids you need. As for monsters, you are better off buying metal monster miniatures from Reaper Miniatures, not only do they look better, it actually winds up being a lot cheaper that way.