The aliens have a vaguely similar body structure to humans, enough that they can convincingly disguise themselves. However, they have blue skin, bulging eyes, and no lips. It is said that they adapted their bodies to look humanoid for the invasion of Earth, therefore what is shown in the movie might not be their true form or perhaps they just don't have any and take the appearance of the dominant species of every planet they take over. This alien species does not have a specific name; in the end credits of the movie two actors playing the aliens are referred to as "Male Ghoul" and "Female Ghoul", while in the source material short story Eight O'Clock in the Morning and its comic book adaptation Nada (which the movie was based on) they are named the "Fascinators".
At some point by the late 20th century, these aliens arrived on Earth and attempted to take over the planet. However, rather than outright invade, they opted for a more subtle approach. The aliens managed to quietly teleport into a series of underground tunnels and take advantage of a television broadcasting station, designing the satellite dish to broadcast false signals that affected human senses on a subliminal level, specifically covering subliminal messages such as "MARRY AND REPRODUCE" and "OBEY" hidden in seemingly ordinary places such as billboards, posters, and magazines, all of this meant to keep humanity subdued for their benefit.
At some unknown point, a small group of humans began to figure out that things were not what they seemed to be. They began to form together, creating small resistance groups which hoped to figure out a way to expose the alien presence to the world. To see the world for what it really was, they managed to develop a pair of sunglasses which were immune to the affects of the broadcasts, allowing people to see the true faces of the aliens and the hidden subliminal messages.
A drifter stumbled across the aftermath of a resistance group after a meeting was split up by the aliens. He managed to find a box of the sunglasses and decided to start trying to fight the aliens himself. After some exploration he decided to return to the alleyway where he found the glasses earlier, after a 10 minute fight scene, he managed to gain support from another construction worker, and eventually the two join up with a resistance group. Unfortunately, the meeting was interrupted by a group of aliens, and in the process many of the people involved were killed.
However, the drifter survived and using stolen technology managed to infiltrate their hideout, working their way to the roof of the broadcasting station. He was killed by aliens disguised as police, but not before shooting and destroying the device that was broadcasting the messages, causing both the subliminal messages and the true faces of the alien to become visible to the general public. Their fate afterwards is unknown, although it's possible that they go to war with the humans.