Q. What is the 'Read-only Community String'?
A. The SNMP Read-Only Community String is like a password. It is sent along with each SNMP Get-Request and allows (or denies) access to device. Most network vendors ship their equipment with a default password of "public". (This is the so-called "default public community string".) Many network administrators will change the community string to keep intruders from getting information about the network setup. This is a good idea. Even if it's only read-access, SNMP can divulge a lot of information about the network that could be used to compromise it.
If there's a "read-only community string", you might expect that there is a"Write community string". You'd be correct. There is also a SNMP Set-Request, which is a command to set certain SNMP MIB variables (e.g., certain OIDs) to a specified value. These writes are protected by the write community string (which should never be set to 'public'!). Many SNMP-speaking devices also have IP address filters that ignore requests (read and write) unless the source address is on an access list.
There's also a SNMP Trap, which is an unsolicited message from a device to an SNMP console (for example, InterMapper) that the device is in an interesting state. Traps might indicate power-up or link-up/down conditions temperatures exceeding certain thresholds, high traffic, etc. Traps provide an immediate notification for an event that might otherwise be discovered only during occasional polling.