Depending on what you paid, the receipt may be a whole form with a bunch of account numbers (that's what you get from a bank or post office), a page torn from a preprinted booklet, filled with hand (small lumberyards, small repair shops) or the fiscal strip (any other retail).
This strip is the proof that you were charged the PDV (engl. VAT - value added tax). Doesn't matter whether you bought a laptop or had a coffee in a cafe - this little strip printed on thermal paper connects your purchase with the tax apparatus of the state. The printer which produced it is connected via modem somewhere with the tax authority's servers and the shop/cafe is charged with the 20% (or thereabouts) of the amount in due PDV. The PDV that the shop paid when buying stuff is discounted, so they really pay it only on the difference, provided they have receipts to show. It's rather nasty, when you think of it, and everyone's paying a bit at every step of the process, way before the product meets the consumer.