Classes, if implemented, would be able to be chosen the second you spawn. Once chosen, they cannot be changed. Classes would allow you to create and use certain things. For example, the warrior class would allow you to excel in the most powerful weapons and heavy armor, but would cause you to be weak in agility and other things.
There would be two types of classes you could pick.
Your Primary Classes, such as Warrior, Archer, and Pirate.
Then there would be your Secondary Classes, such as Cook, Engineer, and Harvester.
For crafting, you will be able to craft a forge using 30 stone, then use that forge to craft swords and such. Woodworking tables would be used to craft parts for siege weapons such as catapults. Those parts would go into your inventory, then you would be able to assemble them anywhere. No use making a catapult in your base when your enemy is across the sea.