In terms of critical infrastructure and key resources (CIKR), an asset is a person, structure, facility, information, material, or a process that has value. For example, in the transportation sector, a bridge would be an asset.
A network is a group of related components that interact with each other or share information to perform a function. For example, a light rail system that crosses multiple jurisdictions in a large metropolitan area would be considered a network.
A system is any combination of facilities, personnel, equipment, procedures, and communications integrated for a specific purpose. For example, the U.S. interstate highways comprise a system within the transportation sector.
A sector consists of a logical collection of interconnected assets, systems, or networks that provide a common function to society, the economy, or the government. For example, the transportation sector consists of vast, open, accessible, interconnected systems, which include the aviation, maritime, pipeline, highway, freight rail, and mass-transit systems.