When you go to the feed store you'll see the chicken feeds labeled something like this.
Starter, both medicated and non medicated is for starting chicks. Usually 18 percent for layer breeds and 22ish for Broilers. If you ever go with Ducklings you want the Broiler starter or Gamebird starter.
Grower, this is for the stage between the chick stage and before they are laying eggs. It is usually between 15 and 17 percent protein. For the sake of convenience some feed companies combine their starter and grower. The name brand feeds will list the ages but for the most part you feed grower or starter/grower until 18 weeks old when layers are getting ready to lay.
Layer, This is for female chickens that are laying or about to lay. Most feed companies recommend switching to layer anywhere from their first egg to about a month before hand so they can build up the calcium levels in their body. You do not want growing chicks eating this on a regular basis as the higher levels of calcium are not good for them.
All Flock, This is for people with mixed flocks of various ages and/or various species such as Ducks, Guineas, Turkeys, etc. So if you have roosters and broody hens with baby chicks this is a popular option. You can also use Starter or Grower for this style of feeding as well. Keep in mind it is very important to free feed Oyster shell so your laying hens can get the calcium they would otherwise be getting in Layer feed.