Depends on the context of the conversation.
When people talk about a DAC for HEADPHONE, is a dedicated box, STEREO ONLY, designed to deliver the best audiophile fidelity to a pair of headphones, from a PC but not limited to it.
When people talk about DAC in an external sound card, saying such sound card has a DAC is really superfluous, as without a DAC, an external soundcard would not have a reason to exist, one wouldn't pipe analog signal to an external soundcard. The reason at all for an external souncard is that an INTERNAL soundcard is susceptible to the inside of a noisy PC. An external soundcard can do surround.
Honestly I think you newbies are sucked in by all these fancy add-ons, to me sound built into modern Motherboard are pretty decent, specially if this is your first rig and you are listening to compressed MP3 anyways. But oh well, your money.