The heart of a thermos is a pair of bottles, one inside the other, sealed together to create a vacuum between the two layers. This flask is usually made out of glass, but it may also be metal or plastic. An outer layer helps protect and insulate the inner flask.
A vacuum flask maintains a vacuum between its two layers in order to minimize heat transfer. The space between the two layers keeps heat from traveling via conduction, and the lack of air prevents convection currents from forming to carry heat away. Radiation is still possible between the layers, so manufacturers generally coat the inner sides of the vacuum flask with a reflective material to minimize this transfer.
Glass is usually chosen as the material for the vacuum flask due to its insulating properties and the ease with which a glass vessel can be completely sealed. Glass flasks are vulnerable to breakage, however, and a small scratch or chip in the inner surface can lead to structural failure. For this reason, many thermos flasks use aluminum or another metal as the main construction material.
Properly sealed, a thermos can keep its contents warm for around four to six hours, or keep its contents cool for 12 to 24 hours. Opening the flask allows some heat transfer, however, so the temperature will be maintained more efficiently if the vessel is kept closed.