What is the difference between a kosher turkey and a fresh turkey?

i don't think there is one, unless the fresh turkey is not kosher.

They are quite different. Kosher turkeys, like any kosher poultry, must have been slaughtered in a specific way mandated by halakha (Judaic law). Subsequently, they can only be cleaned using cold water, and are soaked, then salted in order to draw all remaining blood out of the bird.

Due to the use of only cold water, kosher turkeys usually have residual pin feathers that will need to be plucked before roasting the bird.

Due to the salting, they do not need to be brined as fresh turkeys do. Brining a kosher turkey will result in an over-salted bird.