Hollywood Nuns

In Hollywood, people expect certain traits of nuns:

Dress Code: They will wear the full "penguin suit" habit most of the time (though there are a few exceptions; e.g., an all-white habit). Young nuns-in-training might wear just the headpiece, to let the audience know they're still in the pupal stage.
Temperament: They will either be terrifying spectres of discipline, able to strike fear into even the toughest men, wielding yardsticks like samurai wield katanas, or sweet and nurturing with overtones of forbidden sexuality.
Age Group: They will either be old and super-traditional or young and naive (most of them old).
Names: They will often have "Sister Mary..." (referring to Mary, the "Holy Mother") as part of their name (but not always).
Nun vs. sister: Most of them will referred to as nuns even though most orders in which the women are out of their community are of religious sisters, with nuns living in cloister.

In real life, of course, there are as many varieties of nuns as there are branches of Christianity. Except for a few rare old-fashioned orders, even Roman Catholic nuns, the most commonly stereotyped in media, rarely wear the full habit anymore (see the Dead Man Walking example, below). They usually just wear the headpiece and a plain grey dress, and some orders don't even require that much and have their sisters wear plain, modest street clothes (usually the teaching and/or service oriented orders, for example the Sisters of Mercy and the Sisters of St.Joseph). Evey order also has its unique habit, whether or not it is still worn, and not all of them are black and white. They have different temperaments, but many viewers who grew up in Catholic schools may remember the "scary nuns" most vividly. Orthodox nuns usually wear all-black habits, never of "penguin" color.

Furthermore, not all nuns are Christian. There are nuns in Buddhism called Bhikkhuni or Dge long ma, as well as Hindu nuns, but these are almost never shown in Western media.

A sister trope to Nuns Are Spooky, Nuns Are Funny, Nuns-N-Rosaries, Naughty Nuns. Contrast: Nuns Are Mikos

Examples of Hollywood Nuns include:

Comics

The monstrous Heroic Sociopath Marv, of Sin City, claims to have feared and respected the nuns at his school.

Film

Sister Act at once plays this straight and averts it, which is the source of most of the comedy. Incidentally, Whoopi Goldberg's character takes the name of "Sister Mary Clarence" while in the convent.
Sister Mary Stigmata ("The Penguin") in The Blues Brothers plays the "elderly, scary disciplinarian with a yardstick" role almost straight, except for the fact that she moves like she's on wheels.
The two nuns in Doubt fit the two different temperaments pretty well.
- But their order wore bonnets instead of the stereotypical squarish habits.
The nun in Dead Man Walking, considering this film is based on a true story, is a subversion in dress with her wearing regular clothes with only a gold cross lapel pin to denote her affiliation. When a priest calls her out on that, she quotes the Pope who ruled that the full habit is not necessary anymore as long as a nun wears something distinctive to mark her calling.

Live Action Television

Nuns appear in the series Father Ted on various occasions, varying from strict disciplinarians to sweet/tempting.
Pushing Daisies had nuns in full habits, but in keeping with its setting in a candy-colored Retro Universe, the habits were blue and white instead of black and white.
In an early Season 2 episode of The A-Team, the A-Team traveled to Ecuador to help a group of nuns (including Face's former One True Love from college) in full stereotypical-looking habits protect their mission from a bunch of nuns. Of course, the full habits helped Face and Murdock sneak into the mission to do reconaissance work. Also, Face's old flame had taken the name "Sister Theresa" and she fit into the second temperament.
One episode of House fully subverts this trope when a nun is found to be dying from a strange allergy. The convent she lives in only use the full habit on special occasions; the other nuns who visit her are dressed realistically in conservative dresses with headpieces. The sick nun herself was found to have had an abortion and used an old form of birth control that left a piece of copper in her body (which she was allergic to).
Sister Peter Marie in Oz doesn't wear the habit, but seems to generally follow the rest of the trope (although being rather jaded due to being around the worst humanity has to offer).
Averted with Sister Peg of Law and Order Special Victims Unit. She looks to be about middle-aged, doesn't wear the habit at all, and is neither naive nor excessively strict—which is reasonable for a nun who mostly works on aiding prostitutes to keep them safe and healthy.

Stand Up Comedy

The scary disciplinarian nuns tend to be a staple for comedians who grew up going to Catholic school.