Ohana is a restaurant in the Poly. At dinner games for kids are featured (coconut races and hula hoop contests). The meal is served to you - not a buffet - but the menu is set. There are unusual but yummy side dishes set out first. Then your server brings huge skewers of shrimp, beef, turkey, and pork. You say what you want and how much. It's all you can eat. Cost is about $24.99 for adults and half that or so for kids. We enjoyed it.
I haven't been to the luau in a while, but I remember it featuring much more entertainment - singing and hula performances, etc. Also it was held in a special place outside. More expensive than Ohana but fun.
Maybe others can add to this. Hope it helps.