Reference links:
IETF RFC 3410 - Introduction and Applicability Statements for Internet Standard Management Framework (SNMP) - Dec 2002. An information RFC that covers the basics of SNMP. See also RFC 3411 (Architecture), 3413 (Applications), 3416 (Version 2 of the Protocol Operations), and 3418 (Management Information Base) as well as a few others in the 3400-3499 range.
Simple Network Management Protocol (Wikipedia) - Covers the basics and links to additional reference works.