What is the difference between flooring, subfloor, underlayment and joists?

Working from the very bottom. The Joists are 1st and this is what the subfloor is on generally wood but, If you have a concrete floor this is usually considered your subfloor tjis suports your flooring. Then comes the undelayment section which cushions your flooring, the the actual floor itself i.e laminate etc

Joists are the beams on which the subfloor sits. Joists are approximately 2-3 foot apart, and run the length/width of a room, between them is nothing but space. So the subfloor boards ie chipboard are placed in top and fastened to the beams to create a subfloor base, you can now walk on a floor! Underlay can now be placed on top of the sub floor, for eg carpet underlay, before the carpet is laid. If concrete, this is your subfloor, in many cases ie new build houses it needs a screed to provide a suitable floor smooth and sound for your flooring choice.

Commonly used subfloors in UK are wooden. Those types are suspended floors , supported over a rows of joists - constructional part of subfloor which takes the load (usually wooden beams, can be also aluminium beams). Subfloors are made of variety materials , from mostly known in past wooden floorboards to plywood or chipboards through cementous - suspended concrete subfloors to concrete slabs - poured directly over ground in a past today over PIR insulation with or without under floor heating . Mainly all floor coverings needs subfloor however in some cases the top floor coverings are creating a subfloors itself such as solid wooden floors laid directly over wooden joists , fixed by "secret nailing" or by "small head" floor screws . Very often floor coverings require to have underlay between them and subfloors but that is not crucial as many floor coverings can be glued directly to a subfloor with or without special preparation such as applying additional layer of plywood , priming, additional insulation, latex levelling etc. Although underlayment often provides insulation for top covering itself and those are made with all types of polyurethane foams - of high or low density, chipboards , mixtures of composites, MDF or HDF, rubbers etc. Floor coverings are commonly divided on: - wooden : hardwoods solid floors , engineered floors , softwoods , laminate floors - carpets, - tiles, - concretes, In short , from bottom - joists than subfloor floorboards , than underlay and floor covering on top...

The joist is what is used to support a wooden floor in a house which is called a subfloor. The underlayment is fitted on top of the subfloor and assists with noise reduction, improves comfort underfoot, smooths out minor undulation improving stability and provides insulation. The flooring of choice is then installed. Where WPC (Wood Plastic Composit) or LVT (Luxury Vinyl Plank) is installed underlayment may not be necessary as long as the subfloor is sufficently level i.e. freshly screeded or boarded.