It works with receivers which can be connected to the internet, either by ethernet cable, or in some cases a serial lead. There will be a card sharing server on the internet and your receiver, which must have card sharing software installed, will connect to this.
There are different types of server, the commercial ones, where you pay a monthly or yearly fee for access to several official subscriptions, or the small private ones, normally groups of like minded hobbyists, who bring their own subscription to the party.
The best receivers for this are the Linux based ones like the Dreambox, but it is now possible with a whole range of other receivers.