"Martian" refers to an inhabitant of Mars, the fourth planet of our solar system. While Mars does not support life in the real world, it has long been speculated that it may once have done so. A popular trend of science fiction is the idea of Martians invading the planet Earth, an idea first brought to life by 19th-20th century British author H.G. Wells in his ever-popular story, The War Of The Worlds. Since Wells' original story, the concept of Martian invasion has been re-used and re-imagined in many works of fiction, and the Martians themselves have been envisioned in a variety of forms.
This article refers only to H.G. Wells' original concept of Martians.
The original Martians imagined by H.G. Wells have been described as "creatures composed entirely of brain" and seem to resemble giant octopi, with bulbous, rounded bodies featuring lipless, v-shaped mouths, great disk-like eyes and sporting a series of whip-like tentacles. Their body shape seems highly impractical and one can only imagine how or why they evolved into such a form, but they are extremely intelligent and technologically advanced, capable of constructing tremendous machines to serve as their "bodies". They do not communicate verbally (at least not with humans) and may be telepathic. The only vocalisation they have been known to make is "ULLA!", which may have been some kind of battle-cry.
In The War Of The Worlds, the Martians - having seemingly exhausted their own planet's resources - launch an invasion of Earth. They come by launching great cylinders towards Earth that carry invasion parties and serve as war factories and bases during their occupation. After landing, they make no attempts at communicating with humanity and simply begin slaughtering people en masse. They construct giant, three-legged fighting machines that march into human cities and destroy everything in their path with their terrifying heat-ray weapons. They also use a horrific chemical weapon known simply as "black smoke" that instantly kills anyone who inhales it. In a matter of days, humanity's armies are all wiped out and the Martians take over, seeding the land with a bizarre form of vegetation called "red weed" that spreads across the land like a cancer. They also start rounding up humans in order to feed on our blood, injecting it directly into their own veins.
However, the Martian invasion is doomed from the start. Mars is a sterile world devoid of bacteria while Earth is rife with all manner of microscopic organisms. Having lived for so long on a world without disease, the Martians have no biological defence against any of the bacterial organisms native to Earth and soon their entire occupying force dies off.