If you've got old-style stamps that don't have barcodes on then you need to swap them out for free with Royal Mail in exchange for the new ones. If you try and use any old ones that you've got lying around at home, the person you're sending the letter to will end up with a surcharge to pay...
There's been a massive increase in counterfeit stamps being sold and attempted for use this Christmas - they look unbelievably close to the real thing, but they're really easily detected when the letters are sorted. You can also feel the difference by hand too.