You stick the card into the card reader, the electronics reads the info on the card and if you are authorised to watch that programme it decrypts it so you can watch it.
Where to buy cards - well that depends what you want to buy / watch..
Dunno about
downloading channels but if you google
you will find all sorts of websites listed. You will need to hunt for a few that you like / carry the channels you are interested in
HOWEVER, the sites that carry sport come and go regularly - the pay tv providers dont like their content being provided for free on the internet and go after the sites regularly.