I've been going through a collection of "sci-fi classics" during the summer, which is actually made up largely of cheesy b-movies, but it's not exclusively b-movies in the strictest sense. Some of them (First Spaceship to Venus, Voyage to the Prehistoric Planet, and Voyage to the Planet of the Prehistoric Women) were foreign movies deliberately re-edited to cover up any sign that they were originally mainstream productions originating from the Soviet Union. Others are just really bad (Horrors of Spider Island), some are hilariously bad (Santa Claus Conquers the Martians), and others are somewhere in the middle.
The funny thing about older b-movies is that it seems to be very rare to see one that actually delivers what the title promises. Teenagers From Outer Space is a good example, which has several men from outer space but all of them are clearly over the age of 20. Attack of the Monsters had some monsters who spend most of their screen time fighting each other and not really attacking much. Queen of the Amazons had a grand total of one "Amazonian" woman. Horrors of Spider Island had very few spiders and most of the danger came from one character getting bit and somehow mutating into an invincible monster. She-Gods of Shark Reef had a tribe of women and a reef with sharks that was never named, but definitely no obvious "She-Gods".
When one brings up the subject of b-movies, there are a few images that usually tend to come up. Often they're associated with the 1950's. Common elements associated with them include black and white photography and cheesy special effects. There are plenty of those, like the normal-sized lobster that threatens the protagonists of Teenagers From Outer Space or the pie pan flying saucers in Plan 9 From Outer Space, but the basic definition is a bit more complicated than that.
B-movies are so called because they are usually made with b-list actors. These are lesser known actors that are able to work for less, as opposed to the much better-known a-list stars you see in big budget Hollywood productions. Such films are usually cheap (hence the cheesy effects) and not released as widely as other productions. Though they are often associated with science fiction, a b-movie can really fit into any genre. With the collection of "Sci-fi Classics" I've been going through, there have been several (i.e. Queen of the Amazons, She-Gods of Shark Reef, Horrors of Spider Island) where I have been left wondering what exactly the science fiction aspect of the story was.
There are patterns that seem to emerge in storylines as well as with films of this period. One idea that seemed to be ludicrously common was for a group of (usually all male) characters to encounter a fictional world populated entirely by scantily clad women. Naturally, nobody ever questions this and the subject of how a society in which men do not exist is able to reproduce and thus keep themselves going is never addressed. Aliens often appear in either really unconvincing outfits or as humanoid, and if the latter is picked expect that they will have a language that is by total coincidence identical to English so they will have no trouble communicating with humans.
While the term often implies mediocre quality, not all b-movies are terrible. The H.P. Lovecraft Historical Society's adaptation of The Whisperer in Darkness could technically be considered a b-movie. It was made on a low budget with a cast made entirely of b-list actors and was even shot in familiar black and white. Despite all this, it still managed to be a chilling rendition of a great story that may have even been more Lovecraftian than Lovecraft's original vision.
While hearing the term "b-movie" often brings up images of those from the 1950's, it's hardly confined to that point in history. The concept of the b-movie originated near the end of the silent era and it still continues today. They may have changed over time, but they do still exist. The Whisperer In Darkness was released in 2011 and could be considered a b-movie, but there are still plenty that are as bad as those of the 1950's, if for different reasons.
Modern b-movies work a bit different from the 1950's variety. A lot of them seem to be a bit more fast-paced, usually bringing in large amounts of painfully unconvincing CGI. The plots I have found often involve various monsters that the protagonists (who are often super-attractive but have no real personality) have to survive against while the supporting cast gets killed off with varying degrees of gore. Right now, The Asylum appears to be one of the major producers of modern b-movies, with films like Mega Shark vs. Giant Octopus, but the one that is probably best known at the moment would be the cult hit Sharknado.
I myself have actually been involved with the production of a b-movie. Back in one of my high school film classes, we were given the assignment to create a b-movie of sorts. It was certainly a difficult experience, I can tell you that, largely because I was the only one who actually took the project seriously and wanted to make a real horror film.
I remember several times trying to make the case that our best bet was to refrain from showing the monster but nobody ever listened. My contributions were mostly behind the scenes, doing the camera work and editing that clip from Angel Heart to look like it was being played on an old TV. Unfortunately in keeping with the b-movie spirit somewhere along the line my initial concept for how to handle that scene got lost. I wanted to include the part where Robert De Niro actually bites into the egg as foreshadowing for what was to come later on in the main narrative, but the guy responsible for editing the project didn't end up doing that.
It certainly has the spirit of an old b-movie, however, with the wooden acting and cheesy effects. The bizarre plot fits in more or less with the kinds of things you might see in a b-movie. Basically, the story amounts to a group of junkies dragging their friends into school on a weekend to reclaim confiscated drugs only to get chased by a "soul sucker", and this one girl just happens to have found an old book that somehow explains everything. The "soul sucker" is of course basically just a creepy old man (played by a fairly young man in a wig), with plenty of contrived death scenes for the leads.
In most respects, it is a pretty cheesy movie, but it does have the feeling of an old-fashioned b-movie. Given that was what we were assigned to do, it did succeed in that regard as silly as the final product was. As you can see, b-movies are very much an important part of film culture, as bad and painful as some of them might get. They have been with us since the silent era, and as long as the medium of film exists, b-movies will likely continue to exist in some form or another, even if they change with time.