A southbound interface decomposes concepts in the technical details, mostly specific to a single component of the architecture. Southbound interfaces are drawn at the bottom of an architectural overview.
Northbound interfaces normally talk to southbound interfaces of higher level components and vice versa.
These terms are generic in the sense that they are uniformly used over all layers of an application, i.e. independent of the fact that the system is about hardware, GUI, middle-ware etc.
A northbound interface is typically an output-only interface (as opposed to one that accepts user input) found in carrier-grade network and telecommunications network elements. The languages or protocols commonly used include SNMP and TL1. For example, a device that is capable of sending out syslog messages but is not configurable by the user is said to implement a northbound interface.
B-NT: Broadband-Network Termination. A specific type of Broadband CPE used in DSL networks.
CWMP: CPE WAN Management Protocol (the subject of TR069 standard).