Originally Posted by HillRider

Remember "cables" consist of two items, the housing and the inner wires. As noted both are specific to either brake use or shifter use and should not be interchanged.

Shift housing is "compressionless" and has the reenforcing wires more-or-less parallel to it's length. Is it does not change length when bent in a reasonable radius to make index shifting reliable but does not have great burst strength as it doesn't need it. Shift housing is most commonly 4 mm outside diameter.

Brake housing has it's reenforcing wires wound in a tight coil which give it the essential burst strength. It's typically 5 mm in outer diameter.

Shifter inner wires have a single end and are smaller in diameter than brake wires, typically 1.1 or 1.2 mm.

Brake inner wires are thicker, typically 1.5 or 1.6 mm in diameter, and have two end shapes, one for road bikes and one for MTB use. Many brake wires come with a different fitting at each end and you cut off the unneeded one.

Good quality brake and shift housing is lined with a smooth plastic or teflon liner. Brake and shift inner wires may have a teflon coating but this is not essential and can cause troubles with slippage at the pinch bolts if it isn't scraped off at that position.

Shimano and Jagwire are good brands for both wires and housing.