A chamber pot is the proverbial pot to piss in. Way back in the old days, before we had toilets that automatically dispensed disposable covers and flushed by themselves, people had to use less dignified means to do their business. In rural areas, it wasn't as big of a problem, of course — there's always places to go. But in urban centers, people went in chamber pots, which were, well, pots. They were generally emptied into the public sanitary sewers of the day — that is, the street.
A brief history of waste
The cities of Harappa and Mohenjo-daro in ancient India are the oldest known examples of public sewer systems, dating to around 2600 BCE. At the time, the concern was simply conducting sewage away from cities; toilets were built into houses on the street side, and water was poured down to clear them, making them the earliest version of the flush toilet. Toilets even existed on the second floor of some dwellings, using terra cotta pipes. Waste was washed into open brick-lined drains along the street, which drained into rivers. This system provided at least some degree of hygiene; while the sewers in these cities were open, it was at least partially separated from the public street.
Rome's management of sewage was more effective. More famous are Rome's aqueducts, which carried water over great distances to provide for the city, including its legendary public baths. Similar technological sophistication existed for handling wastes; the Romans initially built covered sewer systems to handle storm water, but later they accommodated waste as well. By 100 CE houses were legally required to directly connect to the sewer system. The latrines associated with public baths were even more sophisticated than those in homes; the multi-seat facilities had wastewater from the baths flowing continuously to flush the waste, which then entered the sewer system and flowed into the Tiber. Laws were passed to discourage people from leaving their waste in the streets; for visitors, large vases in the streets helped maintain the city's cleanliness. Other Roman cities had similar public works; even garrisons in Britain, at the distant edge of the Empire, included primitive sewers.
It's all downhill from there
But when the Roman Empire fell, the shit hit the fan, so to speak. During the Middle Ages, Europe's cities became filthy; the accepted practice for relieving oneself was to simply go in the street — and little changed during the Renaissance. This habit gave rise to instructions in etiquette books regarding what to do when you saw someone you knew relieving themself (ignore them). Europe also gave up on the Romans' love of personal cleanliness; needless to say, public baths were not available in European cities. Waves of disease were routine matters in big cities, and many of those diseases were spread by inadequate sewage collection systems. Little improved until the nineteenth century. In fact, it wasn't until the twentieth century that cities could sustain their populations without immigration, as disease killed off their inhabitants faster than they could reproduce.
This was the golden age of the chamber pot. Though people would usually go in the streets, chamber pots were kept under beds for use during the night. Most were ceramic; many had lids to keep the smell down (though one imagines that they were probably of limited effectiveness.) The wealthy had special cupboards to hold their chamber pots, often with velvet cushions (and of course servants to empty the things.) Some chamber pots were intricately designed, or inscribed with pithy messages. "Break me not I pray in your haste, for I to none will give distaste" says one pot dating from 1651. These interesting features make chamber pots popular collectors' items nowadays.
The end of the chamber pot
In the mid-nineteenth century, sewer systems began to be built in Europe's cities. A cholera epidemic in the 1830s led to the construction of a sewer system in Paris, beginning in 1850. A few years after, a similar system was built in London. At this point, Europe's cities finally began to approach the cleanliness of Rome nearly two thousand years earlier. With actual sewer systems in place, it became possible to build flush toilets, which began to be installed in the latter half of the century. As the streets grew less filthy, disease began to be less endemic to the urban condition, which is one of the factors leading to the explosion of cities in the decades around the beginning of the twentieth century. Indoor plumbing rendered the humble chamber pot unnecessary in the industrialized world; while they are still used in places without indoor plumbing, in the First World this once ubiquitous feature of life has become largely obsolete.
Much of this knowledge is thanks to http://www.sewerhistory.org/; much of the rest is shit I learned in my urban planning classes. A picture of the pot quoted above can be found here (http://www.liverpoolmuseums.org.uk/nof/craftndesign/access/shop.asp?ant=household&antlookup=chamberpot).