Bicycle frames are constructed from materials which place emphasis on being strong, lightweight and stiff to aid power transfer. Aluminum, titanium, steel and carbon are widely used in all levels of frame production.
Brake and gear cables are traditionally attached to bicycle frames with lugs on the outside of the tubing, this is known as external routing.
Recent advances in technology have allowed manufacturers to route the cables through the inside of the tubing, this is known as internal routing. Internal routing gives a very clean appearance, can increase the stiffness of the frame due to additional channels and in some cases may provide aerodynamic benefits too.
For certain applications, including full suspension bikes and cyclocross bikes, manufacturers use an outer cable along the full length of the cable to protect the inners from mud, water or accidental damage, this is known as full outer.