There are two kinds of feed lines that I have seen. Coaxial and "ladderline".

Ladderline is a pair of parallel copper wires that are separated by a space in between. This is either air or a plastic called a "dielectric".

When used correctly, ladderline is a cheap way of feeding an antenna with extremely low loss, however, it is not always possible. Ladderline can be used to feed a dipole, or a G5RV but should not be used to feed an "Off Centre Fed dipole".

Coaxial is a single copper wire (signal) which is surrounded by some plastic (or air or foam) also called dielectric. The dielectric is then surrounded by a mesh of wire which is then surrounded by a piece of protective plastic. The wire mesh is often connected to the earth so that the signal wire does not receive interference when receiving. During transmission, the mesh prevents the transmitted signal from leaking out (radiating), which is a good thing if you are running high power near where people are likely to be.