Arduino Uno Rev3 Internal Quartz
The ATMega328P chip is, indeed, clocked by a ceramic resonator. The website is misleading in stating that it has a crystal (I have emailed the webmaster to try and get this corrected, though I don't hold out much hope).

The 16MHz crystal in the schematics is part of the USB interface circuitry and has nothing to do with the timing of your running code. It is purely used to clock the ATMega16U2 chip which provides USB connectivity (USB requires a higher accuracy for stable communication).

There are other boards available with better crystals - most often boards that have native USB connections, such as the Arduino Due or Leonardo, or any of the myriad of boards by other manufacturers, like Teensy or chipKIT.