All are ways to obtain information collected locally from a remote device / location.
SNMP get and sets counters and other information, in it's most basic sense. It's standards based, very extensible and almost ubiquitous in the networking world.
RMON is a standard for remote monitoring and more attuned to capturing and analyzing traffic, or characteristics of traffic. SNMP accomplishes most of the basic stats (volume, status, etc.) RMON is more up the protocol stack for analysis.
SAA is now know as IP SLA (see http://www.cisco.com/go/saa) It is the current incarnation of technology that evolved from tools Cisco built into IOS to measure metrics critical for assurance of IBM data center technology (SNA) when that was a much bigger piece of the networking landscape.
NetFlow is a means of monitoring and measuring IP flows. It can be used for rudimentary troubleshooting but is more commonly used for traffic management and integration with Operational Support Systems (e.g., billing) in my experience. See http://www.cisco.com/go/netflow.
The latter two are Cisco-specific, although I believe there is effort to incarnate them in IETF RFCs.
All these methodologies generally (but not entirely) require a tool to query and present the data for human consumption. SNMP tools are very widespread and built into some OS's (i.e., mibwalk). Open Source tools are also available for much snmp functionality (e.g., MRTG / RRDTool).
Hope this helps, please rate helpful posts.
