Gearbox
Most manual gearboxes have 5 or 6 forward gears and one reverse gear. The gear ratios are different depending on the type of car and the engine characteristics, performance cars can have longer gearing so that cars can get to 80+mph in second gear, whereas small town cars will have much shorter ratios.
Manual gearboxes take a lot of stress and quite often drivers with poor gear changing technique can put extra strains on the components of the gearbox. Poorly maintained gearboxes can suffer from worn out components and seize up, so its important that a gearbox is maintained correctly and the oil level is maintained.
Gearbox makes whining sound
When the car is in gear there's a loud whining sound coming from the gearbox. The sound will stop when the clutch is pressed and the gearbox is disengaged. The sound can increase in volume and pitch as the speed increases.
Gearbox temperature warning light showing on dashboard
There is a light similar to the one pictured here showing up on the dashboard. Different manufacturers sometimes have slightly different warning light symbols but it should be similar to the one pictured.
Gear changes are difficult
Changes between gears have become difficult, the gearstick no longer moves freely between gears, it sometimes seems to get stuck or won't go into a gear properly.
Gearbox makes grinding sound
When changing gear the gearbox makes a loud grinding noise. This will be a more common occurrence on cars with manual gearboxes but can also happen with automatics. There can be various reasons for it but it always presents as a loud, unpleasant sound of mechanical scraping/grinding.