What is that flexible pipe I saw used on Holmes on Homes to replace interior copper piping called?

I was watching HGTV a wile back and Holmes on Homes came on. In this home he had to run all new plumbing inside the home and used this piping that kinda looked like a garden hose but of course it wasn't. Now I'm in a situation where I may need to run a new hot water line in my house to replace an old one. I'm getting no hot water to my master bath (probably a blockage) and the pipes are so old I'm afraid that if I have an acid treatment done it'll just eat through already worn parts of the pipe. I'm fairly good with plumbing connecting new pluming fixtures & repairing lines, as compared to the average home owner that is. My main problem is knowing what to buy to begin with. After that I'm usually ok.