It is a stamp, you can buy traditional stamps or if you go to a post office they have started using these. They are large stickers that look like the usual stamps, but once they are 'stamped' by an official post office printer it then becomes a 'stamp'. I believe they are using them to reduce the need for stamps of different denominations. In the past anything that wasn't standard first class required multiple stamps of different denomintions - now they can just print the correct 'postage paid' on the same standard sticker.