Helical focusing is a very common manual focusing mechanism. The tube containing the lens elements is fitted inside a collar, to which it is coupled by a coarse screw-thread. The focusing collar is held captive in relation to the camera body, and the lens-tube is prevented from turning, so that when the collar is turned, the lens-tube is made to move forward or back relative to the camera body.
Because of the shallow pitch of a screw-thread, helical focusing makes it easy to move the lens in very small increments. In order to offer sufficiently fine adjustment over the whole focusing range (which is in at least the overwhelming majority of cases arranged over a single rotation of the focusing collar), helical focusing typically offers only moderately close focus, and other measures (e.g. accessory bellows or extension tubes) are needed to focus closer.
Helical focusing provides 'unit focus'; that is, the whole lens assembly moves (in contrast to front-element focusing, widely used on cheaper cameras where the lens allows it). A helical mechanism can be used with scale focusing (i.e. with no focusing aid other than an engraved scale), with a ground-glass screen, or it may be coupled to a rangefinder.
A Helicoid is an extension tube with built-in helical tube length elongation facility. It can be used to make enlarger lenses usable as camera lenses. That means lenses with diaphragm but without own means of focusing. Some helicoids additionally serve as adapter for M42 lenses on M39 screw mount or on CSC bayonet mount. With help of a small M42-to-M39 reducer ring these specialized helicoids might take most enlarger lenses since most of the latter are for M39, but many for M42 screw mount. Enlarger lenses with long focal lengthes need normal extension tube(s) added to the helicoid.