The Badlands are the lands between the tunnel from Castle Discordia to Castle Town and Le Casse Roi Russe. As the name suggests, it is a rough patch of land with very few living things on it. Even devil grass fails to grow on parts of it. The temperature also tends to be fairly cold.
Roland Deschain and Susannah Dean were forced to cross the Badlands with inadequate clothing. They were cold, but were in no danger of freezing. Its effect was insidious, making them miserable and irritable. They knew they were reaching the end of the Badlands when the devil grass began again and they saw King's Rooks.