Artificial Intelligence, or AI/Ai, is the intelligence of a machine and the programs/instruments needed/used to create the machine capable of Ai. Ai is the a field which studies the creation of intelligence agents (an intelligence agent being a machine which can negotiate its environment and react in ways which insures its success at the task at hand).
Essentially, people in Ai are specially trained in the creation of machines (typically robots/computers) which can maneuver the environment they are put in to perform a specific task. These machines are considered intelligent based upon the fact that they can perform "behaviors" which humans consider to be intelligent.
Behaviors which are considered intelligent are: understand human speech, learning, manipulation, motion, creativity, the ability to perceive, and planning.