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.
Speaker level or line level depending on the particular components available.

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 ..