In terms of the network management model, a network management station (NMS) is one that executes network management applications (NMAs) that monitor and control network elements (NE) such as hosts, gateways and terminal servers. These network elements use a management agent (MA) to perform the network management functions requested by the network management stations. The Simple Network Management Protocol (SNMP) is used to communicate management information between the network management stations and the agents in the network elements. NMS is described in RFC 1157 "A Simple Network Management Protocol".
An NMS provides FCAPS functionality for the whole network. FCAPS: Fault, Configuration, Accounting, Performance, Security, are the categories defined by the ISO model. In non-billing organizations accounting is sometimes replaced with administration.
Looking into the industry aspects, there are various players in the market like NokiaSiemens - NetAct, IBM - NetCool. Open source NMS solutions are also available like OpenNMS.