Seeds unlock the ability to farm different types of crops and orchards. Once a seed is obtained, you are able to infinitely plant the corresponding crop/fruit in any crop field/orchard. There is no benefit to purchasing multiple seeds of the same type: seeds are not consumable and can be 'planted' in as many fields/orchards as you like.
On Easy and Medium difficulty, you start the game with a number of seeds. The type is determined by the map seed. On Hard difficulty, you do not start with seeds.
More seeds can be obtained from certain traders at the Trading Post for 2,500 units each.
Seeds cannot be planted until spring.
List of available seeds