A thermos bottle helps to keep the temperature of your beverage inside cold or hot because it is insulated against heat loss (or heat gain) through the use of double wall construction. Essentially the thermos bottle works in much the same way as the double paned windows of your house. They use the air that is trapped within the two walls of stainless steel to help minimize the transfer of heat.
This concept is highly innovative and has been with us for a very long time and innovative ideas to come up with better do exist but the traditional style remains very popular. The outer skin is usually a durable flask coat which can take knocks but the inner skin is fragile and if damage does occur it is usually the inner that breaks and this renders the flask useless. Depending on the design it is possible to replace a broken inner.