Well the host plants are those specific plants that the butterflies lay their eggs on. When the eggs hatch, they are the larva or caterpillars
and will eat that plant until they are ready to form their chrysalis (pupa stage).
Nectar plants are abundant and will support those butterflies that nectar, and some also are used by the hummers. Not all butterflies nectar, some feed on sap, dung, etc. But that is more than you want to know.
www.dallasbutterflies.com has a list of host plants for butterflies under butterfly gardening. The list is extensive, but you can see on there what I am talking about. You may also get a feel for how over-whelmed I felt trying to organize the forum.
Oh, I read again your question. To keep it simple put host plants as those the caterpillars eat, and nectar plants for the adult butterflies.
