IMHO,
Stand-alone DACs:
- a stand-alone DAC takes digital input from S/PDIF and/or AES/EBU and outputs analog.
- a stand-alone DAC with a USB interface is the same as above, but with added circuitry for taking input through USB. Doesn't include other traditional "sound card" features like ADC. This is probably what is mostly meant by "USB DAC".
Sound devices, sound cards, audio interfaces, etc:
- designed specifically for computer use. In addition to DAC conversion can include features like ADC for inputting analog audio, digital audio outputs, digital audio inputs, adjustable headphone outputs, mic preamps, etc. Some typical variants:
- Internal: PCI, PCie
- External: USB, FireWire (aka IEEE 1394)
- Built-in on motherboard