Blocks or Cubes are natural cubes that spawned in on map creation. Player crafted items that can be placed, for the purposes of this game and wiki, are instead known as "structures". Most blocks can be harvested for resources; many can be placed back down.
Technically a block and a resources are different files (ex Clay and Clay Cube) with the block and resource each having different admin spawn codes. When its placed in the world, its a block; when its been mined and is in an inventory, it is a resource. As they usually share identical icons and similar in-game descriptions, as far as players are concerned, they are the same thing.