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Fiberglass batt insulation.
Fiberglass batt insulation is still the preferred choice for residential construction. It is not necessarily the best type of insulation around and it is virtually not used in commercial and industrial construction. Fiberglass batt insulation used to come with a paper vapour barrier attached to one side. It would fit between the joists and had a reinforced paper edge for stapling the batts in place. Today the standard fiberglass batts are slightly wider than the space between the wall studs or ceiling joists and are installed by friction-fit. Instead of each batt having its own vapour barrier, the barrier is now installed in wide sheets covering the insulation as well as the studs or ceiling joists. Batts are pre-cut to fit lumber spaced at 16" or 24" centers. Fiberglass insulation is also available in continuous rolls of up to 25 feet (7.6 m ) in length.
Compressing the insulation reduces its effectiveness. Cutting it to accommodate electrical boxes and other obstructions allows air a free path to cross through the wall cavity. To prevent this from happening all outlet boxes have to be wrapped with a vapour barrier before installation. The airtight R-2000 outlet box is more effective, see vapour barriers.
Also, gaps between batts can become sites of air infiltration or condensation (both of which reduce the effectiveness of the insulation) and requires strict attention during the installation.
Fiberglass batts can be installed in two layers across an unfinished attic floor, perpendicular to each other, for increased effectiveness at preventing heat escaping.
Batts can also be used to insulate a floor above an unheated crawl space. This can be a very unpleasant task and I speak from experience.
Some years ago, I helped my son to re-insulate the floor underneath his home. Laying on our backs we first removed the old insulation. Next we added 2"x3"s to the 2"x6" floor joists and stapled a 6 mill (heavy duty) polyurethane vapour barrier against the floor and sides of the joists.

We then installed friction-fit R-20 batts of fiberglass insulation which had to be held in place temporarily by driving nails into the sides of the joists. Once that was done we installed sheets of galvanized diamond mesh (also known as stucco wire mesh) to prevent the insulation from sliding down and the mice from coming in (the old insulation was riddled with mice nests). While doing the job, laying on our backs and working directly above our faces, we were showered first with debris falling from between the floor boards while nailing on the 2x3s and later by insulation particles. Needless to say that wearing proper eye protection and face mask is a must.