What will happen if I give my Arduino Uno more or less than 5 volts?
Will it break at &lt; 5V? Not likely.


Check the datasheet for the IC's on the board and find the minimum power supply voltage the device works at.
At typical ATmega328 Safe Operating Area (under "Speed grades" in the datasheet) specifies approximately 4.1V for 16MHz.
If you require to use the USB interface, you'll have to check the datasheet for that chip too.


Will it run at &lt; 5V? Maybe.


If the supply voltage gets below the minimum operational voltage, the controller may behave erratically. That means it can do anything (un)expected. This value is in the datasheet and probably somewhere around 2.7V.
If the Brown Out circuit is configured (in fuses), then the controller will switch itself off when supply voltage gets too low to prevent undefined situations. Once the supply voltage returns, it restarts.


Will it break at > 5V? Depends on where you apply it.


V(IN) should work fine for 7-20VDC as it is fed through an on board regulator. Notice though that the regulator has to dissipate the excess voltage and can get pretty hot when you draw a fair bit of current. The regulator will fail if the voltage goes over 20V or gets too hot too often;
+5V should work fine up to 5V, and I personally wouldn't worry if an external supply delivers up to 5.5V unloaded. Above that 5.5V you will probably release the magic blue smoke.