SNMP stands for SimpleNetworkManagementProtocol It can be used for WebTesting, HttpUnit. http://net-snmp.sourceforge.net
SNMP is a UDP-based protocol, originally designed for configuring and monitoring routers, gateways, and similar equipment. (It can run over TCP, though this is not commonly done).
It has sense been used in many other applications.
Despite the name, it hardly is "simple". It is pretty simple, except for ASN.1 (AbstractSyntaxNotationOne) and OIDs, binary encodings, and the plethora of slightly different versions, and the routing and security issues.
SNMP is a client/server protocol, where a SNMP manager (usually an automation system, or an operator's workstation) issues commands and queries to one or more agents (network nodes, etc.). In addition, the agents can issue asynchronous traps to the manager.
SNMP has a small command set (get, set, get multiple, etc.) and a numerical hierarchical namespace of ObjectIdentifiers, or OIDs for short.