The term information system has the following meanings:

1. A system, whether automated or manual, that comprises people, machines, and/or methods organized to collect, process, transmit, and disseminate data that represent user information.

2. Any telecommunications and/or computer related equipment or interconnected system or subsystems of equipment that is used in the acquisition, storage, manipulation, management, movement, control, display, switching, interchange, transmission, or reception of voice and/or data, and includes software, firmware, and hardware

Source: from Federal Standard 1037C and from MIL-STD-188 and from the National Information Systems Security Glossary

The simplest model that describes the Structure and Behaviour of an Information System takes five objects:

For Structure:

• Repositories, hold data permanent or temporarily, such as buffers, RAM, hard disks, cache, etc.
• Interfaces, exhange information with the non-digital world, such as keyboards, speakes, scanners, printers, etc.
• Channels, connect repositories, such as buses, cables, wireless links, etc. A Network is a set of logical or physical channels.

For Behaviour:

• Services; provide value to users or to other services via messages interchange.
• Messages; carries a meaning to users or services.

Source: from book "Seguridad de la Informacion", 2004 ISBN 84-933336-7-0

3. In the mathematical area of domain theory, a Scott information system (after its inventor Dana Scott) is a mathematical structure that provides an alternative representation of Scott domains and, as a special case, algebraic lattices .