dvbd ("dvb daemon") is a traditional UNIX daemon that sits between the DVB cards and the DVB software. It (in theory) takes control of all your DVB cards, and then any program wanting access connects through dvbd instead. This has the advantage that multiple programs can connect to dvbd, whereas only one program at a time can normally use the DVB hardware.
This means that dvbd can tune a card to a given frequency, and then make available all those channels to any programs that connect, effectively allowing you to watch/record multiple channels at once, providing they're all transmitted on the same frequency.
If you have multiple cards it will tune additional ones as necessary to provide access to as many requested channels as possible. You can schedule recording too, and give them a priority, so for example if you're watching a live broadcast and a high-priority scheduled broadcast begins, your live stream will stop, the card will tune to another frequency and your recording will begin. Of course if you have two cards and only one was in use, dvbd is intelligent enough to use the second card for the recording so you can keep watching your live broadcast.
At least this is how it's all supposed to work in theory - unfortunately development seems to have stopped, as not much work has been done since 2004 :-(
However since 2011 there is an updated version of dvbd containing some fixes and improvements (see external links below).