SNMP monitoring in SCOM is an example of agentless monitoring.
Since SCOM 2012 there are a lot of templates related with SNMP devices, before SCOM 2012 you had to install the xSNMP management pack, nowadays a lot of vendors are included in SCOM 2012 by default.
Here is a list of well supported devices: http://www.microsoft.com/en-us/download/confirmation.aspx?id=26831
When you start the discovery wizard to add a new network device, you are creating a new discovery rule, keep this in mind.
To add a new SNMP monitor, go to Administration --> Discovery wizard and execute the network device wizard.
This wizard has some important points to take into account.
In the first screen it will ask you for general rules, where you have to set a name for the discovery rule, a description and finally, very important, it will ask you for the resource pool.It is strongly recommended to make a special resource pool for the network devices, especially for security reasons related with password or SNMP community distribution.
The wizard will ask you for the discovery method, select whichever you need.
Default accounts for network devices could be SNMP v1,v2,v3, you have to create one if you need to monitor your devices via SNMP, it isn´t mandatory, but if it is possible do it in order to have accurate monitoring.
Now wizard will ask you about devices, you can enter here the IPs of the devices and whether you want to use SNMP or just ICMP.
Finally it will ask you for the discovery schedule, it refers to when the rule is going to be executed to detect changes in the device configuration. It depends on your environment, but usually once a week is enough.
This wizard as we said, creates a new rule in Administation --> Network Management --> Discovery rules, if you want you can execute this discovery rule manually from this menu.
Basically this management pack monitors the device state and configuration, for example if you are monitoring a switch, it is going to monitor VLANs, CPU, memory performance and interface statistics.
Some vendors has a more specific management packs for their devices and they can monitor more accurately than the default SNMP management pack, but usually the information provided by the default management pack is enough to have a global view.