Difference in brakes and switcher cables?
Standard brake cables are 1.6mm thick and have either a cylindrical end perpendicular to the cable for MTB-style levers, or an inline stepped-cylinder (like a pear) for road-style levers.

Standard gear cables are 1.2mm thick and have a smaller cylindrical end, inline (cable enters circular face).

By standard, I mean both Shimano and Campagnolo use these gauges and end-styles and I'm not aware of any other variety used by gear or brake levers in current popular use...