1. Surveillance refers to covert, continuous or periodic observation of persons, vehicles, places or objects to obtain information to assist in criminal investigations. 2. There are two main types of surveillance - static surveillance which involves continuously watching a location or person, and mobile surveillance which follows a subject as they move from place to place. 3. Surveillance can be conducted on foot, using vehicles, from air or using technical means like audio/video equipment. It plays an important role in gathering evidence and information for crime investigation and prevention.