If a manager wants to gather the current status of a device, it doesn't have to wait for an incoming SNMP trap (which may never come if the device has completely failed). SNMP managers can send GET requests to "get" a value from a device. Depending on the data needed, your SNMP manager may send a GET that demands the device's on-board temperature reading, whether it has any alarm conditions to report, or just about any other information. Upon receiving a GET, an SNMP agent response with a GET response that contains the requested data.
Your manager has an overarching "bird's eye" view of your network, so it's a great place for automated decision making. Imagine that your SNMP manager is made aware (via traps or GET responses) that several devices in one server room are overheating. Your manager can be configured to make an immediate, automated decision to activate your backup air-conditioning (HVAC) system. To do this, it must send a SET message to "set" a value on the target device (the backup HVAC, in this case). A SET-response (see the pattern?) is sent by the agent to confirm that it has received and followed the SET instructions. There are a few other types of SNMP messages, but you understand all the basic concepts if you understand traps, GETs, and SETs.
Where can I go for more SNMP information?
Obviously, this hasn't been an exhaustive discussion of SNMP. I've tried to give you a reasonable introduction to the protocol without bombarding you with techno-jargon. There's a lot more information about SNMP available right here on the DPS website. I recommend: