I just read the fuses of my arduino uno and the clock source is set to "external crystal oscillator" even when there is just a quartz crystal on the board. Same for an arduino micro. Why is this like that?
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Thanks for the explanation 
I got confused and thought about these integrated crystal oscillator circuits with both the quartz resonator and oscillator circuitry inside.
That is a bit confusing since I have seen many processors where you can use an internal oscillator, external crystal or external clock (like he mentions) that has a square wave out you feed into the processor (often used as a master clock on a board for other things). When I see "external crystal oscillator", I tend to think along the lines of one of these guys:
What resource lists that fuse configuration as "External Crystal Oscillator"?
It is listed as "low power crystal oscillator" or "full swing crystal oscillator" in datasheet. Many sources call it "external crystal", which I think is a better description - but saying that the clock source is "low power crystal oscillator" is accurate, since there circuitry to make an external crystal oscillate is included on the chip. For using an external crystal oscillator, you use the "External Clock" option.
DrAzzy:
What resource lists that fuse configuration as "External Crystal Oscillator"?
Online fuse calculators call an external crystal "Ext. Crystal Osc."