As a result of a fault in certain 1TB Seagate hard drives found in October 2009- July 2011 iMacs, Apple has started a program where you can get your hard drive replaced free of charge if it is found to be faulty.
Note that you should already know about this if you are affected as Apple has already sent out emails to those concerned. However, Apple as always allows to to double check if you’re affected: simply enter your serial code on the page, and it will tell you.
If you are one of the unlucky few, then the replacement will be carried out for free by Apple themselves or an Authorized Service Provider.
It is very unlikely you have a problem though, and even if you do, it’s pretty minor.