How is a subwoofer connect to a 2 channel system?

Since I've never ventured into higher end systems than my current one that uses a multi-channel receiver, I'm interested to find out how people hook up subwoofers in a 2 channel system. I know subwoofer often have speaker level inputs and there are subwoofer specific amplifiers available, but I don't know much more than that. I see 2 channel systems running dual subwoofers and wonder how they are setup. This is just a fact finding mission to increase my knowledge.
Is that just hooked up in parallel with the main speakers or do most amps have a dedicated sub output?
For powered subwoofers, connect the subwoofer amp to a tape out or a line out ( if there are more than one set) on your pre-amp.The internal subwoofer crossover cuts out all the mid and above frequencies .
George - Can't use "Tape Out", no volume control from pre-amp. My particular Integrated Amp has a subwoofer line out.
So do most folks just use a single driver sub for a two channel system. My integrated has a sub out (one for right channel and one for the left channel). So both channels feed into one single driver sub ..