Simple Network Management Protocol (SNMP) is a part of the Internet Protocol Suite.  SNMP is used in network management systems to monitor status of devices and also spot problems. It consists of a set of standards for network management, including an Application Layer protocol, a database schema, and a set of data objects.

SNMP exposes management data in the form of variables on the managed systems, which describe the system configuration. These variables can then be queried (and sometimes set) by managing applications. It is a standard for managing devices such as routers,switches,servers etc.

Overview and basic concepts 
In common SNMP usage, there are a number of systems to be managed, and one or more systems managing them. A software component called an agent (see below) runs on each managed system and reports information via SNMP to the managing systems.

SNMP agents expose management data on the managed systems as variables (such as "free memory", "system name", "number of running processes", "default route"). But the protocol also allows active management tasks, such as modifying and applying a new configuration.
The managing system can retrieve the information through the GET, GETNEXT and GETBULK protocol operations or the agent will send data without being asked using TRAP or INFORM protocol operations.  Management systems can also send configuration updates or controlling requests through the SET protocol operation to actively manage a system.  Configuration and control operations are used only when changes are needed to the network. The monitoring operations are usually performed regularly.

RFCs 
 RFC 1155 — Structure and Identification of Management Information for the TCP/IP-based Internets
 RFC 1156 — Management Information Base for Network Management of TCP/IP-based internets
 RFC 1157 — A Simple Network Management Protocol (SNMP)
 RFC 1441 — Introduction to version 2 of the Internet-standard Network Management Framework
 RFC 1213 — Management Information Base for Network Management of TCP/IP-based internets: MIB-II                             
 RFC 3410 (Informational) — Introduction and Applicability Statements for Internet Standard Management Framework
 RFC 3411 (Standard 62) — An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks
 RFC 3412 (Standard 62) — Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)
 RFC 3413 (Standard 62) — Simple Network Management Protocol (SNMP) Application
 RFC 3414 (Standard 62) — User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)
 RFC 3415 (Standard 62) — View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)
 RFC 3416 (Standard 62) — Version 2 of the Protocol Operations for the Simple Network Management Protocol (SNMP)
 RFC 3417 (Standard 62) — Transport Mappings for the Simple Network Management Protocol (SNMP)
 RFC 3418 (Standard 62) — Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)
 RFC 3584 (Best Current Practice) — Coexistence between Version 1, Version 2, and Version 3 of the Internet-standard Network Management Framework
 RFC 3826 (Proposed) — The Advanced Encryption Standard (AES) Cipher Algorithm in the SNMP User-based Security Model

References 

Internet protocols
Internet standards
Network management
Application layer protocols