What is a forever stamp?
A forever stamp is a stamp that is valued (sold and cancelled) at the current local letter rate of postage. For example, if the current cost of local postage is 50 cents, the forever stamp is valued at 50 cents. If the price of postage rises (or miraculously, falls) the value of the forever stamp would be the new value. The forever stamp is forever valued at that rate.
So, before the price goes up, buy a lot of forever stamps and you will save money. Forever stamps also save having to use 1 or 2 cent stamps to cover the postage increase. Saves you and the post office time and money.
Yes, you can still use a Forever stamp issued by the United States Postal Service in 2009. This was the revolutionary idea of the Forever stamp - you can purchase a first-class stamp at today's price then not have to purchase additional postage later when the price goes up. The stamp is called the "Forever Stamp" because it will be valid forever for a first class envelope.