Hardware or software decoder?
The old standard for DVB cards was to have a hardware MPEG decoder on the card itself. That would decode the incoming MPEG stream, therefore not use the CPU resources (but more PCI bus resources). These cards are often called full-featured Cards.
Nowadays, most cards are budget, i.e. they don't have that additional chip. Instead, the still encoded, unchanged stream goes to the CPU and some viewers software then does the decoding.