Unsigned ints (unsigned integers) are the same as ints in that they store a 2 byte value. Instead of storing negative numbers however they only store positive values, yielding a useful range of 0 to 65,535 (2^16) - 1).
The difference between unsigned ints and (signed) ints, lies in the way the highest bit, sometimes refered to as the "sign" bit, is interpreted. In the Arduino int type (which is signed), if the high bit is a "1", the number is interpreted as a negative number, and the other 15 bits are interpreted with 2's complement math.
unsigned int ledPin = 13;
unsigned int var = val;
When variables are made to exceed their maximum capacity they "roll over" back to their minimum capacitiy, note that this happens in both directions
unsigned int x x = 0; x = x - 1; // x now contains 65535 - rolls over in neg direction x = x + 1; // x now contains 0 - rolls over
Corrections, suggestions, and new documentation should be posted to the Forum.
The text of the Arduino reference is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the reference are released into the public domain.