As mentioned elsewhere I 'volunteered' to write something we can point poster to when then have yet another problem with baud rates and the like. It's still work in progress and will get finalised over the next couple of days.
Comment welcomes.
clawson 
Very nice!
I might say that this:
As the name suggests this oscillator is inside the AVR and requires no external components. The
word ‘calibrated’ is misleading though as for many applications, especially UART communications,
the frequency it runs at is not accurate enough for reliable operation. Which means you really need
to be using...
... should perhaps be a bit less absolute w.r.t. suitability of RC oscillator for comms. In very many instances, it is plenty good. Maybe just say "often not accurate enough".
It depends, of course, on the device itself. Some recent devices ship which a +/-3% factory calibration. An important caveat is that the factory calibration is generally at a specific VCC and temperature.
It's worse for some older devices. The m8 for example has a +/-10% factory calibration, but only the 1 MHz calibration is loaded at reset, even if the device is running from the 2, 4, or 8 MHz oscillator. Newer single-oscillator devices with CLKPR and CKDIV8 avoid this.
You may also want to mention ceramic resonators, which generally include the capacitors in the same package.
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