First appearing in 1897 in a short story called The Crystal Egg, and 1898 in the novel The War of the Worlds by H. G. Wells, the first invasion from outer space novel. The Martians of Wells are described as "intellects, vast and cool and unsympathetic" and when first seen by the unnamed narrator of the story are described thus:
A big grayish rounded bulk, the size, perhaps, of a bear, was rising slowly and painfully out of the cylinder. As it bulged up and caught the light, it glistened like wet leather. Two large dark-colored eyes were regarding me steadfastly. The mass that framed them, the head of the thing, was rounded, and had, one might say, a face. There was a mouth under the eyes, the lipless brim of which quivered and panted, and dropped saliva. The whole creature heaved and pulsated convulsively. A lank tentacular appendage gripped the edge of the cylinder, another swayed in the air.
The Martians, who come from a dying world seeking to occupy the Earth as a new home, and use the blood of animals and humans as a food source.
To do this they rely mainly on large war machines that move on three legs, making them look like tripods, and attack using an intense heat ray and a rapidly growing alien organism, known as the Red Weed that quickly kills Earth native plant life and which seems to be making the atmosphere more like that of Mars.
Earth defenses having proved powerless against the invaders the day is saved then both the Martians and their Red Weed are killed by a common Earth bacteria.
While Wells never used his Martians again, the last we hear of them being a dying Martian calling 'Ulla-ulla-ulla' at the peak of Primrose Hill in London, they have however been used by a number of other writers.
The Martians in Comics
In comics they have mostly appeared in adaptation of the novel by Wells.
Martians incredibly similar to H.G. Wells' creation had a prominent part in Marvel’s series Killraven detailing a future in which the Earth is controlled by the Martians after a second successful invasion.
The Martian invasion of the late 20th century is a complete conquering of Earth, with the death of all of Earth's heroes, including Spider-Man, the Avengers and the Fantastic Four. This event is the main catalyst of the 616 universe diverging into the 691 universe of the Guardians of the Galaxy of the 31st century. This reality is even called "The War of the Worlds."
After their successful invasion, the Martians return home and create a memorial museum, only to be stricken by a plague, killing every member of the race with the exclusion of Ripjak. Mars is quarantined for centuries after the plague.
Other Comic Appearances
They play a significant part in The League of Extraordinary Gentlemen Volume 2 where they begin their invasion of Earth. Instead of common Earth bacteria killing them however The League deliver a deadly virus created by Dr. Moreau.
Boom! Studios Second Wave series follows an ordinary man, Miles Walker, during the Martians second invasion of Earth.
In 1953 The War of the Worlds was released, a loosely based adaptation of Wells' novel. Instead of the Martian tripods the invaders now used indestructible flying machines to wreck havoc upon the Earth.
In 2005 Steven Spielberg directed an adaptation of War of the Worlds staring Tom Cruise. It is less accurate to the book than the 1953 film, it is set in modern day America, there is no mention of 'Martian' or Mars in the movie and they are likely to be from another unknown planet.
There was also a series based on a TV series called the War of the Worlds which lasted for 2 season between 1988-90. The series has very little to do with H.G. Wells original story however since the plot and Martians are a continuation of the 1953 movie adaptation.