thanks for putting that link on ! Apparently Tesco do it by car reg recognition , they get to know which of their stores you visit by clocking the reg, so we will all have to stop going to Tesco or black your reg out driving into Tesco Car park
Nice theory Miss Marple, but it's actually done using their CRUCIBLE database. By presenting your Clubcard when paying for goods, you announce yourself to Tesco, allowing them to track your name, address, purchase history to Tesco. They then use the ZODIAC tool to do more advanced profiling (like "how many Glossop Tesco shoppers who live in SK6 visited our store during w/c 1st August 2011, and what was their average spend").
If you don't like it, options are : boycott Tesco, shop there but don't present your Clubcard, or the really sneaky option is to swap your Clubcard with a friend in another town in order to screw up their data.