To give the word decibel more context, the decibel scale is relative one, so you have to specify a reference point. In home and car audio, when we say decibel, we usually mean dB(C), and that C is for C weighted scale. The C weighted scale basically measures loudness when things are easy to hear, like big speakers and subwoofers. If you get a SPL meter, you will see a switch that says A and C, keep it on C when you want to test your speakers/subs. A weighting dB(A) is used for measuring quieter sounds and is generally used for gauging noise pollution, and it is using a different reference point than C.
There are other scale weightings aside from A and C, but they aren't widely used.
By the way, decibels is a logarithmic scale, so increases and decreases in power in a way similar to the Richter scale. In case you have forgotten math class, here is a scale that demonstrates how it ramps up:
The bottom row is the ratio of the top row, obviously, and the top row is dB. So, for example, if you want to double your decibel power, you have to add 6 dB, you can see where 6 is right over 2. If you wanted only a tenth of the loudness you have now, you will have to turn the volume down by 20 dB. Here is that scale in a graph, where it might make more sense for you:
Clipping is something I think is easy to understand when visualized. Here is a good picture describing clipping:
Like the picture says, clipping is when the signal is boosted up so much that there is not enough channel capacity to encompass it all, so the signal peaks get clipped, like Isiberian said. The audibility of the clipping depends on the severity of the clipping, in my experience. If your signal is clipped a little bit, you probably won't notice, but if it clipped a lot, it sounds horrendous. If you are really curious, you can download something like
audacity, load up a sound file, and keep boosting the loudness of the sound file until your available wave capacity is maxed out, and then keep pushing it for a harder clip. Pretty soon it sounds like hell. Sometimes some bands like death metal and punk deliberately clip their recordings for a hard thrashing, edgy sound.
To add to Isiberians descritopn of distortion, there is different kinds of distortion. The THD listed on AVRs stands for Total Harmonic Distortion. Every speaker and subwoofer will add distortion, mostly harmonic, to the output, and if want hi fidelity you want to keep that distortion as low as possible. Here is a good short video that explains it succinctly:
[video=youtube;FzeZbJceKZE]http://www.youtube.com/watch?feature=player_detailpage&v=FzeZbJceKZE[/video]
