Yes and no. Every external sound card that connects to your PC via USB or SPDIF and has an analog output for connection to headphones or an amplifier, contains a DAC.
However, that external sound card may also contain other features, such as multichannel Dolby/DTS processing, other digital sound processing, or other features such as a mic input or a speaker amplifier.
The term DAC, as it is used on head-fi and most audiophile forums, refers to a more specific device that is intended to perform ONLY the digital-to-analog conversion, without the additional features and functions that might be included in an "external sound card". The audiophile DAC is usually limited to stereo processing, and is usually designed to maximize sound quality by using audiophile components and circuit designs.
So - you CAN use most external sound cards to perform the same function as what we call a "DAC", and you can use an audiophile DAC as your PC's primary sound device. HOWEVER, the audiophile DAC will probably not have all the same features and functions as the typical PC external sound card, but it might be capable of producing higher quality sound. Of course, there are exceptions on both sides of this - external soundcards with excellent sound quality, and audiophile DACs with many of the functions of PC soundcards.
Does this help?
