Short answers, pulled from the story.
In No Exit, "Hell is other people" refers to Sartre's philosophical concept of the look: the experience of being seen and judged by another consciousness, which forces you to perceive yourself as an object rather than a free subject. Garcin cannot escape Inèz's judgment of him as a coward, and that permanent, inescapable gaze becomes his torment.
No Exit premiered at the Théâtre du Vieux-Colombier in Paris in May 1944. The original cast included Michel Vitold as Garcin, Gaby Sylvia as Estelle, and Tania Balachova as Inez, directed by Raymond Rouleau.
The three characters are Garcin, a journalist executed for deserting his country during wartime who also mistreated his wife; Inèz, a postal clerk who seduced her cousin's wife and caused the cousin's death; and Estelle, a socialite who killed her illegitimate child and whose lover then committed suicide.
Huis clos is French for closed door and is the equivalent of the legal term in camera, from Latin meaning in a chamber, referring to a private proceeding held behind closed doors. English productions have used titles including No Exit, In Camera, Vicious Circle, Behind Closed Doors, No Way Out, and Dead End.
The first Broadway production of No Exit ran for three weeks in 1946 at the Biltmore Theatre and was directed by John Huston, with Claude Dauphin as Garcin and Ruth Ford as Estelle.
No Exit has been adapted into several films, including a 1954 version directed by Jacqueline Audry and a 1962 version directed by Tad Danielewski. Composer Andy Vores created a chamber opera based on the play, which had its world premiere on the 25th of April 2008 at the Boston Conservatory's Zack Theatre.