It differs slightly depending on the retailers setup and payment methods, but the basic principle is the same whether its payment or refund. Once the instruction is received and authorisation given, a 'block' is put on that amount so it may look like the transaction is instant to the card holder, but the actual money then begins its journey through the various parties involved, who all add time to the process and take their fee along the way. What appears on your account and what actually happens to the money are not the same.

this is a pretty good flow chart of how it works, and why it takes so long:
http://www.infographicsshowcase.com/...nfographic.png