Basically Ricky, a C-Line is like a URL, it connect your box via the internet to a remote server (usually somewhere like north Africa) which downloads the subscription card details to your box, then when you connect to a pay-channel i.e. Sky Sports it uploads the card details to the satellite when requested which is every few seconds. It enables you to watch many subscription channels for a much lower monthly fee, also you can watch channels from many other satellites such as Hotbird 13E and Astra 1 19.2E with all the main European channels on.
This method of card-sharing albeit illegal is very popular down here in southern Spain with many companies offering the service for around 25€ a month, they even advertise in all the local papers and magazines!
We have more companies now setting-up offering the ITV/BBC channels in standard definition only via 'redistribution' over a private network for between 13-20€ a month.