What a Thermos Does
Most people assume a thermos is used to keep hot things hot. This isn't the exact purpose of a thermos though. A thermos is designed to minimize the amount of heat flow between two environments (i.e. the inside of the thermos and the surrounding outside air). Heat flow isn't just heat loss, it also accounts for heat gained. A thermos helps prevent heat loss when the thermos is holding a hot substance and then helps prevent heat gain when the thermos is holding a cold substance. So a thermos can keep hot things hot and cold things cold.
Thermoses aren't just used to keep your coffee warm throughout the day though, but are also used in labs and experiments. Whenever you have a lab using a Dewar, you are using a thermos. Sir James Dewar, in 1892, first started the use of thermoses. The proper name is dewar, but Thermos (first commercial producer of dewars) has had its name take the place of dewar in the general public. Scientist use dewars in experiments to keep experiments at constant temperatures. Industries also use dewars to hold liquified gases or for flash freezing processes. Whenever a low temperature is desired, a dewar will be used.