Metering, or: what can’t be heard, should be seen
Music is a tricky business and many experienced people will tell you that what sounds good IS good. There are still areas where exact measurements are essential and artistic decisions need to be balanced with the technical ones. Can you imagine a tailor making a cool suit without a measuring-tape? Probably not - Neither do I.
In the early days of analog recording, there was no need for complex measurement. The engineers could get away with level metering on preamps, meter bridge on the console and the tape machine. Digital processing brought in some new rules but also many useful tools that could never work in pure analog. In this article I will focus on the digital in-box metering gadgets.
Peak meter
Perhaps the most basic one of the level meters. It’s sensing the actual level of signal and displays it as a bar. 
Because the signal changes are too rapid to be shown directly, all these meters use some kind of “visual management” to ease the movement of the meter. Peak hold feature and numerical readouts gives good insight about the happening on the channel.
When working with input/output channels, reaching the top of the scale (0 dB) means that the device is clipping and we are loosing information (speaking musically: the sound distorts). However just approaching the zero area can be dangerous, more on this later.
VU meter
Stands for Volume Unit meter, and it differs from a peak meter in it’s signal integration mechanism. That means the measured level is averaged for a duration of 300 miliseconds (typically). This behavior is not only eye-candy, but resembles human listening, so high levels usually mean high volume too.
Mastering engineers often use peak and VU meters simultaneously to determine the actual peak to average ratio of the material. Of course, this doesn’t mean other people cant use them :)
Recommended: PSP vintage meter
Spectral analyzer
Is a bit more complex than the former ones but is also more versatile. It is based on so called Fast Fourier Transformations (FFT) which divides the spectrum (lets say 20Hz to 20kHz) to different bands and their signal level is measured. The amount of these individual bands determines the accuracy in terms of frequency. 
A spectral analyzer can be helpful when dealing with conflicting instruments that need to be targeted with an EQ. I recommend inspecting various sounds and instruments with a spectral analyzer to see where these reside. One can learn more by experimenting and listening than by staring at – many times outdated – frequency charts.
My favorite VST for spectral monitoring is Voxengo SPAN
It’s ability to set up a restricted freq range is very useful, focusing on bass or mids was never easier!
Phase, stereo correlation meters
Are used to monitor the width of stereo-field. 
They are mainly the tool of the mastering guy to inspect unhealthy stereo movement in the low freq ranges when preparing vinyl masters. But as always, what can be done in the end is easier to handle ahead of time :o)
Inter-sample peaks
These are the problems mentioned in the peak meter paragraph. These are perfectly documented on the site of SSL, manufacturer of premium studio gear. To read more and get the X-ISM meter visit: http://www.solid-state-logic.com/music/X-ISM.html
I think many consumer grade players suffer from this phenomena and studios should consider using a meter like this. Another solution for home made recordings could be setting the master threshold to -0.3 dB instead of -0.1 dB
Happy metering!
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