Popper is a slang term given broadly to the chemical class called alkyl nitrites, that are inhaled. It is used for practical purposes to facilitating anal sex by increasing blood flow and relaxing sphincter muscles or for recreational drug purposes, typically for the "high" or "rush" that the drug can create. Poppers have also been historically used for sexual encounters, initially within the gay community.
If you trace the bottle of amyl (a type of alkyl nitrite) through late 20th century history, you trace the legacies of gay culture on popular culture in the 20th century.
Poppers were part of club culture from the mid-1970s disco scene and returned to popularity in the 1980s and 1990s rave scene.
Popper use has a relaxation effect on involuntary smooth muscles, such as those in the throat and anus. Most widely sold products include the original amyl nitrite (isoamyl nitrite, isopentyl nitrite), but also variants such as isobutyl nitrite, isopropyl nitrite (2-propyl nitrite, increasingly, after EU ban of the isobutyl form). In some countries, to evade anti-drug laws, poppers are labelled or packaged as room deodorizers, leather polish, or tape head cleaner.
Ethyl Chloride, sometimes labelled as Maximum Impact and sold in aerosol cans, have also been misleadingly marketed as poppers, despite having no historical relation to poppers and doesn't contain alkyl nitrites.