A baker makes bread as well as other baked goods to sell. In the poorer districts of Panem, few people can afford to buy their products. It is mentioned that people from the Seam can't buy it most of the time.
Each district has its own style of bread. The style reflects the industry of that region. For example, District 4 has bread with seaweed in it because it is the fishing district. Another example, District 11 has bread that is in the shape of a crescent moon with seeds and/or grain on it because it is the district of agricultural crops. District 3 has small bite sized rolls.
Bakers also make and design cakes, this is what Peeta did at his family's bakery which led him to excel at camouflage which was used as an advantage in the 74th Hunger Games arena when Peeta gets injured by Cato, which resulted in Peeta camouflaging himself by a stream until Katniss finds him after the announcement that two tributes can survive if they're both from the same district.