Flexible plumbing is known as tubing. It can be used to connect parts of a plumbing system without the rigidity of hard PVC plumbing. An example would be connecting a submersible water pump in a sump to the return bulkhead going through the bottom of the tank. Another example would be to connect the drain bulkhead to the filter in a sump.
There are several different kinds of tubing. The most common for aquarium use are clear and braided tubing. There are other types of large tubing commonly used for drain lines but they are harder to find at your neighborhood hardware store.
The difference between clear and braided tubing is the braided is stronger and doesn’t kink as easily when bent. Either will work but it may depend on your application as to which will be better.
Tubing is connected to the rest of the plumbing system with fittings called barbs. Barbs will slip inside of the tubing and be secured with plastic or stainless steel pipe clamps.
Tubing can come in more sizes than standard PVC sizes so you need to pay attention to what you are buying. Tubing is measure by O.D. (outside diameter) and I.D. (Inside Diameter). Go by the I.D. when buying tubing to match barb fittings. Try to match them by inserting the fitting into the tubing before purchasing. Tubing is usually sold by the foot.
Barb fittings will have either slip or threaded ends to fit onto existing plumbing. If you are having a hard time getting the barb to go into the tubing, try using a hair dryer or heat gun to heat up the tubing to expand it.