Starter or starter/grower is formulated for what growing chicks need. It is usually medicated to help the chicks build up a resistance to coccidiosis, a disease that is in the soil. It is low in calcium and higher in protein than layer feed.
Layer pellets, crumbles or mash (depends on the size of the feed) is formulated for laying hens. It is not medicated and has more calcium and less protein than starter/grower.
Do not feed layer feed to chicks because the excess calcium can harm them. It is OK for laying hens to have starter/grower, but you need to have some oyster shell available for the extra calcium.
If you have roosters by themselves they should be fed a game cock feed or other feed that is low in calcium. If the rooster is with the hens, they can eat the layer feed (although it is not the best for them).
Scratch or scratch grains is usually fed as a treat or supplement. It is not good as the only feed for chicks or adults.
Hope this helps.
