There is more than one flavor of manual transmission, as technology has provided for more efficient and more driver-friendly shifting over the years. Common in older vehicles is the sliding-mesh gearbox. This one engages gears by moving the main shaft with gears that are located either to the left or to the right of the neutral position. This approach requires that the input and output shaft speeds be matched before the shift can be accomplished. Any inaccuracy in matching the shaft speeds means the gears will not mesh correctly, hence all the negative effects of a noisy and bad shift.
A more modern approach (constant mesh) is used, particularly in HD applications, in which all of the gears are simultaneously meshed turning, but not engaged. Clutches are used to engage the desired gear with the output shaft which means quieter and a less retching shift. While an improvement, there is still the need for skill on the part of the driver to make a smooth shift from one gear to the next.
The final improvement comes with the name of synchro-mesh. By including a conical clutch and several other components, the speed matching is accomplished completely within the gearbox itself.