SNMP MIBs (Management Information Base) describe OIDs (Object Identifiers) for SNMP.
An OID turns something like 18.104.22.168.4.1.412.2.1 into dmtf.dmtfStdMifs.dmtfServiceLayerMIF
This is useful for monitoring applications, as they know what they're looking at. MIBs also include definitions for the kind of data returned by a specific OID, so they know to translate something to Int or DateTime.
Listed below are MIB files for selected models: