HostInfo is a feature of Nelsnmp which allows you to retrieve vendor, operating system and version information from an SNMP enabled device.
Nelmon uses this feature in the nm_check_version plugin in order to provide security compliance monitoring for Nagios in terms of which images are running on a network device.
HostInfo is also used by the Ansible module snmp_device_version.
Currently the support for devices has been limited to the types of devices I have easy access to. If you want support for HP, Juniper or something else feel free to talk to me about it.