Well they all use different scripting languages however probably the easiest to do this is in is using PHP / MySQL.
The best system i've found is called oscommerce which is a free script. If you have a host with cpanel / fanstasico [like mine click here] you can install it automatically else as long as you have PHP /MySQL support
Your very welcome to have a look at the copy I installled if its of interest.
click here -- user: Jon pass: 1234
I'm not sure whether you mean just how would they do it or are looking into making an online shop but this is certainly a good start. Other things to consider are SSL certificates (secure connection for credit card) .. you could start just using paypal :-) Good Luck and sorry if this wasn't really what you meant.!