Zombie Blocks are blocks placed by the player and never naturally found.
( CS - Cobblestone, RF - Rotten Flesh, ZH - Zombie Head )
The Zombie Head can be placed anywhere, the same goes for Rotten Flesh.
Zombie Blocks spawn a Zombie to attack any player / mob that passes it, though it will not attack the player who placed the block. Since Zombies catch fire when it's daytime, it is most advisable to place them in caves or at night because if the spawned Zombie catches fire, it will attack the player who placed it and since Zombies can break down wooden doors, it will make it harder for you to escape if you have one (especially if its almost broken).