Bushings/Bearings are responsible for the smooth operation of your Gears, and are essentially the 'socket' that they sit inside to spin freely. They are rated by their exterior diameter and will need to match the rating for your gearbox (i.e. 8mm Bearings only for an 8mm Gearbox). No an 8mm Gearbox does not mean it takes bigger BBs, this is purely the size of the recess for the bearings to sit inside. Bushings are more simple in design, with the trade off that they are tough and last a long time especially under the user of higher-rated springs, whereas Bearings offer less friction when turning, for a slicker operation. When using Bushings be sure to keep them well lubricated with a thicker Gearbox grease, as they're not as freely-spinning as Bearings. Avoid the use of grease/lubricant on actual Bearings as this can attract grit/dirt and harm operation.