This is a subject that was thrown about in IRC a while ago.
There seems to be a lot of confusion, particularly to the newcommers, about what these things mean and how to measure them.
Couple of basic points to start.
Watts. Watts are the basic unit most panels are sold by. "100 watts" or "215 watts" or whatever.
From there on, it gets just more and more confusing...
So... lets look at a typical panel.
Maximum power: 100 watts.
Open circuit voltage: 22V Short circuit current: 6.14A.
Anyone knows "Power = volts times amps" - so 22V * 6.14A = 135W. Huh??
The answer is, that the "Open circuit" voltage is with no load. As you apply load, the voltage will fall.
And the "Short circuit current" of 6.14A is the maximum current the panel can deliver into a dead short.
It can't provide any POWER in that circumstance of course, because a load resistance of 0 ohms means no work done!
The "maximum power point" is the sweet-spot where the voltage hasn't dropped too far, AND the current is as high as we can manage while delivering a reasonable voltage. The product of the two produces the "maximum usable power". And for this module, that's about 17.5 volts and 5.72 amps (100 watts).
If you were to take a large variable resistor and an amp meter in series and connect them across your solar module under perfect illumination (square on to full sun), you would expect to be able to vary the current from not much (depends on the maximum resistance), up to a little over 6 amps.
If you also connected a voltmeter across the panel while you did this, you would see the voltage change from almost zero (when you measure maximum amps), up to close to 22V.
Simply measuring open circuit volts is almost meaningless.
Measuring maximum (short circuit) current can tell you a number of things about your panels however. If you only get a few milliamps from a large panel like this (but you DO get full open circuit voltage) almost certainly means a cracked, broken or burned conductor in the panel, or a high-resistance connection.