If you are like me, you don't own a car, but occasionally find yourself needing to get from point a to point b, and public transport simply won't do. You *could* buy a car, but if you need to make less than 10 such round trips a month - or if you plan to do it drunk, high, or otherwise unable to drive - then a cab is probably for you. At roughly $1/minute, a cab isn't cheap if you do it often, but then, owning a car has a pretty high initial monthly cost itself.
As for Yellow Cabs themselves, there's not much to say. You can call their number and have a cab sent to you - depending on the time, the night, your luck, and your location, one will show up anywhere from 1 minute to 15 minutes. Usually, you get a phone call when your cab arrives.