Footprinting (also known as reconnaissance) is the technique used for gathering information about computer systems and the entities they belong to.
Passive footprinting
Passive Footprinting is the process of gathering information on a target by innocuous, or, passive, means.
Active footprinting
Active Footprinting is the process of using tools and techniques, such as performing a ping sweep or using the traceroute command, to gather information on a target.