whatis a dac?

I'm new to this game and need some basic advices. I have a b&k 4090. What are they talking about when they say it sounds better using analog input/dac as opposed to digital. I thought digital anything was best!?
A DAC is a digital to analog converter.
A DAC (Digital Audio Converter) is an outboard device that goes between your transport (or your CD player if used only as a transport) and your preamplifier or receiver. It takes digital signals and converts them to analog use so that they will properly interface with your non-digital components. (There are a very few preamplifiers that will accept a digital signal directly.) As for your comment about everything digital being best, there are more opinions on that subject than there are audiophiles. In brief, some prefer vinyl analog because they feel it is more true to the basic harmonics of the music and is often less harsh sounding; others favor the supposedly more accurate signal transfer of digital signals and dislike the possibly greater distortions inherent in an analog signal. Fans of digital sometimes feel that analog signals are more euphonic than live music. As to which input to use for your own listening pleasure, try them both and make your own determination. Not all inputs on all preamplifiers or processors are necessarily equal in terms of the electronic characteristics they possess, nor are the output characteristics of all DACs the same; and thus the experience of one person may not be applicable to your particular situation.
If you are just starting out, my advice is to forget about "analog" (i.e. turntables & LP records) and stick with digital. Once the music is turned into numbers, it can be manipulated, massaged, and transformed in ways we can't even dream yet! It is totally the way to go. I've been buying CD's since 1984 and they keep sounding better as the hardware improves. Sure, the best analog sounds better (for twenty minutes until you have to turn the record over -not counting the five minutes you spend cleaning & preening the darn thing) than your average digital, but the new upsamplers and formats (SACD & DVD-A) are closing the (small) gap daily. Put your money on the future...
Excellent post, Drtmth58. Streetdaddy, you wont hear it any more succinct or accurate than that. You might want to look at the GoodSound website. (http://www.goodsound.com) There is some very good general starting info there, as well. Good luck, Charlie.