Sprinkles are very small pieces of candy used as a decoration or to add texture to desserts. The candies are usually too small to be eaten individually and are in any case not intended to be eaten by themselves, being nearly flavorless, but rather as a topping or coating for ice cream, cakes, or other candies.
Sometimes a distinction is made between sprinkles, which are spherical and can be of any color/flavor, and jimmies, which are elongated, and are usually chocolate. The term jimmy is used mostly in the North East. 
Other shapes of similar toppings exist, but as there are no universal names for these products, they are usually called sprinkles as well. Examples include flowers and shapes appropriate for a specific holiday, such as Halloween sprinkles. On the other hand, toppings which are more similar in consistency to another type of candy, even if used similarly to sprinkles, are usually known by variation of that candy's name (for example, mini chocolate chips).
An interesting alternative use for sprinkles is the confetti cake. In this dessert, sprinkles are mixed with the batter, where they slowly dissolve and form little splotches of color within the cake, the appearance of multi-colored confetti. Confetti cakes are popular for children's birthdays in the United States.