Instance migration in OpenStack can be used to redistribute VM loads or for maintenance and comes in two types - nova migrate and nova live-migration. Nova migrate shuts down instances during migration and does not allow target host selection, while nova live-migration suspends instances to minimize downtime but requires shared storage and has additional requirements. Administrators should use nova live-migration when choosing the target host is important or to minimize downtime, and nova migrate when not choosing the host or if configuration prevents live-migration.