Artificial intelligence (AI) is a broad field, and means different things to different people. It is concerned with getting computers to do tasks that require human intelligence. However, having said that, there are many tasks which we might reasonably think require intelligence - such as complex arithmetic - which computers can do very easily. Conversely, there are many tasks that people do without even thinking - such as recognising a face - which are extremely complex to automate. AI is concerned with these difficult tasks, which seem to require complex and sophisticated reasoning processes and knowledge.
People might want to automate human intelligence for a number of different reasons. One reason is simply to inderstand human intelligence better. For example, we may be able to test and refine psychological and linguistic theories by writing programs which attempt to simulate aspects of human behaviour. Another reason is simply so that we have smarter programs. We may not care if the programs accurately simulate human reasoning, but by studying human reasoning we may develop useful techniques for solving difficult problems.
AI is a field that overlaps with computer science rather than being a strict subfield. Different areas of AI are more closely related to psychology, philosophy, logic, linguistics, and even neurophysiology. However, as this is a CS course I'll emphasise the computational techniques used, and put less emphasis on psychological modelling or philosophical issues. I'll just briefly touch on some of the widely discussed philosophical issues below: