A pressure cooker is a pot with a special lid that cooks food faster by regulating the pressure inside the cooker. With a pressure level higher than sea level, water boils at a higher temperature, and the food cooks faster than in just an open pot.
Pressure cookers are wonderful for cooking beans or rice. They can also cook and tenderize cuts of meat and are great for making stock out of bones. A broiler can be used if at the very end if you want a mallard reaction.
A pressure cooker should generally NOT be used to pressure can low acid foods unless tested and approved by the manufacture. Using it without the pressure gauge for high-acid water bath canning is of course OK.
Pressure cookers are also tricky to use on items that cook rather quickly in boiling water. For example, raman noodles would be overcooked by the time the cooker got up to pressure and then cooled down low enough for the lid to unlock.