The metal part that slides up and down the zipper. Sliders are only required when you are buying continuous zippers, that is you buy the zipper and cut to length and then apply the slider.
Finished zips are just that, a complete zip ready to sew with its slider fitted at the factory.
Sliders comprise on Non Lock, Pin Lock or Auto Lock and they can be double tab, single tab or decorative tab.
A non-lock slider is the simplest slider and is only suitable when gravity is not an issue. This slider would slip down in a trouser application.
A pin lock slider requires the operator of the zip to press the tab of the slider down on the zip to lock it in place (Very common in Upholstery).
An auto lock slider is just that, it has a spring-loaded tab, which automatically lock the slider in place when the slider is let go off.