Most motherboards have built-in sound features. These are often adequate
for most users. However, you can purchase a good sound card and speakers
at relatively low cost—a few dollars at the low end can make an enormous
difference in the range and clarity of sound. Also, these onboard systems tend
to use more system resources, so you are better off with a real soundcard for
gaming.
Sound card quality depends on a few factors. The digital-analog conversion
(DAC) is generally the most important stage for general clarity, but this is
hard to measure. Reviews, especially those from audiophile sources, are worth
consulting for this; but don’t go purely by specifications, as many different
models with similar specs can produce completely different results. Cards may
offer digital (S/PDIF) output, in which case the DAC process is moved from
your sound card either to a dedicated receiver or to one built into your speakers.
Sound cards made for gaming or professional music tend to do outstandingly
well for their particular purpose. In games various effects are oftentimes
applied to the sound in real-time, and a gaming sound card will be able to do
this processing on-board, instead of using your CPU for the task. Professional
music cards tend to be built both for maximum sound quality and low latency
(transmission delay) input and output, and include more and/or different kinds
of inputs than those of consumer cards.