Depends on the horse, some may do too well in grass and need restricting, others might need supplementary feeding due to work being done or loss of condition; then there are some metabolic/laminitic types that can hardly eat grass at all.
Mine gets restricted grazing and is fed hay or haylage year round along with a good quality chaff and fast fibre with a vit/min supplement. The Manual of Horsemanship is a good book to get the basics.
Mine grass, and a large multi mineral block left in the paddock. If grass is in short supply - droought which we seem to be heading for right this moment - Hay, in winter a large round of haylage in the paddock with the top cut off and allowed to eat ad lib.
When I do hard feed it's usually Crushed Barley, Meadow Chaff, Ground Linseed & Sugarbeet
it depends. At grass my lot get the following
The kids:
Vits/mins in usually a balancer only
Pony:
Nothing!
Skinny mare:
Old faithfuls & hifi with vits and mins