A piggy bank is a container used to store small amounts of money in coin form, generally by children. There will be a slot somewhere for the coins to be placed, and possibly a rubber, plastic or cork stopper on the base to allow the contents to be removed.
The stopper guards against two piggy bank clichés; the knife and the hammer. Obviously, without the stopper, it's quite difficult to retrieve coins through the single slot, especially when there are fifty of them in the bank. A long, flat knife blade (such as is found on a butter knife can be inserted into the slot and used as a guide for the coins. It's not uncommon in The Beano and similar titles to see a character concentrating on this tricky and often fruitless task.
Of course, if time is of the essence one may move to plan b; just smash the back and gather your booty from the shards of clay. If you choose this method, tradition compels you to later reconstruct the piggy bank with glue and tape. Good luck - it's not that easy. Yet every cartoon youngster invariably owns a battle-scarred piggy bank that looks not unlike Frankenstein's monster, still faithfully hoarding its loot despite years of abuse.
Piggy banks come in many shapes and forms, but the most common is, of course, the humble pig, with a slot in its back and a stopper in its belly. But! The term piggy bank is actually derived from pygg, a cheap clay used in the Middle Ages. People would store spare coins in "pygg jars". It was not until the nineteenth century that potters started producing homonymous pig-shaped banks. These were immensely popular with children, and remain so to this day. The pygg/pig legacy is still evident today in the salt pig, a large-mouthed open-access salt container found in many kitchens.
(There is another theory as to the origin of the term; that the small, coin-collecting jars were referred to as 'pigs' because they were fed 'scraps' of money until such time that they could be 'butchered' (or smashed).)
In some European countries the notion of the pig as a lucky animal has helped cement the piggy bank's popularity; they are often given as gifts for this reason. Aside from superstition, they are universally popular as a method of teaching children to save. Banks often make gifts of piggy banks to attract young savers.
However, you can make a fair deal of money from a piggy bank without ever dropping a coin in the slot, for they are highly collectible items. Creators of note include Arthur Wood, Ellgreave and Wade. Noders who grew up in the UK in the 1980s may well have owned one or more of Wade's NatWest Pigs, a promotional line for the NatWest bank. If you still have any, don't drop them! They may well have more value than the contents of your 'real' bank account.
The most famous piggy bank is probably Hamm, the John Ratzenberger-voiced know-it-all oinker (coinker?) from the Toy Story series, who lent his name to Debian Linux 2.0.
Inside my piggy bank (I used a knife):