A designated driver
(d.d.) is the "babysitter" for a group
of people enjoying a night
out on the town.
This person (or people) will not drink at all during the night and is responsible for making sure of the well-being of the remainder of the party. It's their job to keep an eye on folks to try make sure no one drinks too much they get utterly sick (although it's ultimately the drinker's choice--the d.d. only suggests it's a bad idea, forcing a drunk is both dangerous and near impossible, plus it puts too much responsibility on the d.d) or suggest they don't randomly go home with someone they just met (unless they told the d.d. ahead of time this was in their agenda).
Most importantly, the d.d. is in charge of making sure everyone gets home SAFELY. Whether this means everyone crashes at the d.d.'s, the d.d. drives everyone to their homes, or in special cases hails a cab for the friend depends on situation. Safety is their most important function.
In any regular group of drinking buddies this job should rotate frequently.