As per the title, ASDA guarantee your shopping to be 10% less than anywhere else (that's Tesco, Waitrose, Sainsburys or Morrisons.
Previously they only offered to MATCH the price
All you have to do is enter in your receipt number at http://www.asdapriceguarantee.com
and they will match the cheapest price, plus give you 10% if it's costs more at ASDA.
You can claim up to £100 per month