Known in the US as sprinkles and in Holland as hagelslag (yes, really), hundreds and thousands are tiny cylinders of artificially colored candy, usually each about half the size of a grain of rice.
Hundreds and thousands are usually sold in mixed colors, but it is also possible to buy them in single colors and in some flavored varieties, notably chocolate. They are usually used to decorate cakes and cookies, but in Holland chocolate hagelslag are sometimes eaten on bread for breakfast along with coffee. This is not the most healthy way to start the day, and it has the added disadvantage of being addictive, since very few things can replace the pure sugar and caffeine boost that this combination supplies.
The main ingredients of hundreds and thousands are sugar, artificial coloring, and sometimes, as noted above, artifical flavoring. I'm sure there are other ingredients in there but, surprisingly, I have been unable to find a comprehensive list anywhere online. If anyone can /msg a list of them to me, I'll add them here. Otherwise I'll have to buy some, and I don't want them in the house; I might start eating them for breakfast.
Many thanks to ThePope for supplying the correct spelling of hagelslag.