DACs or digital to analogue converters are making a comeback thanks to the way so many of us listen to music off a computer. DACs can be used with any digital source and unless that source is pretty serious they generally improve the sound of onboard conversion.
Be it a PC soundcard, DVD player, DAB tuner or CD player you will usually find a coaxial (phono socket) or optical (Toslink socket) digital output to connect to a DAC. This then connects to a hi-fi amp and delivers better sound. More DACs even come with USB inputs