In order to get coolant from one place to another in the engine bay, coolant lines (like the ones pictured at the left) are required. These lines are often made of rubber, and are connected using round clamps on the outside of both ends of the hose.
Coolant lines need to be able to withstand a lot of heat and the chemicals in antifreeze/coolant. Some aftermarket hoses are made out of silicon.
Some coolant lines are metal, but these can only be used where they won't be required to move. These lines are also typically smaller. A common use for these lines is for the transmission cooling, but they typically carry automatic transmission fluid, rather than fluid meant to only be coolant.
Coolant lines must deal with the chemical stresses of heated and cooled chemicals such as ethylene glycol and propylene glycol, as well as rust particles, oxidation, and other contiminants.