Artificial Intelligence is a field of science that has several goals. The first, often called "weak" AI, is the effort to design and implement computer systems that can perform tasks requiring intelligence when performed by humans. AI in computer games, knowledge based systems and such are typical examples of this branch.
The second, less tangible branch, often called "strong" AI, is the research that ultimately aims to build artificial systems that display real intelligence. It does this by trying to understand or model the nature of human intelligence.
Both branches are multidisciplinary, in that they have close relations with the fields of computer science, philosophy, medicine, psychology, biology and linguistics.