As fiction goes, aliens are typically extraterrestrial beings from other galaxies. They are often far more advanced in knowledge about technology. For this reason, they are able to travel to other galaxies. They are similar to humans in the sense that they have goals of expanding their territories by means of colonization and invasion. Like humans, they have a system of hierarchy. They are often pretty much like humans in every way except for the fact that they don't look like humans unless they are disguised. Sometimes, however, aliens are shown to be completely non-human -- perhaps a trancendental intelligence or even some form that's not recognizably humanoid or even biologically bound.
Alien Horror is a descriptive sub-genre -- any horror book which contains, well Aliens of some form or another with the Aliens being of extraterrestrial nature. Alien Horror is often a feature of Science Fiction Horror or Space Horror.
Alien stories can sometimes contain political allegories. An example of an alien invasion book with political allegories would be H.G. Welles' War of the Worlds.
Level of Characterization
Low. Not much need for complex characters.
Level of Plot Complexity
High. Interesting subplots, twists and turns, technical information, etc.
Level of Supernatural
Low. Aliens aren't Supernatural creatures.
Level of Scary
High. High tension and paranoia.
Level of Violence
Space or on Earth.
Science-Fiction shares similar themes such as Aliens and technology. Aliens can be the cause of certain Apocalypse stories. In some stories, Aliens are able to control people's minds, making the subgenre similar to Mind-Control.
Stories set in the future. Themes of war.
1. The War of the Worlds, H.G. Wells. Giant cylinders crash to Earth, unleashing unearthly creatures armed with heat-rays and destructive machines that set their sights on England.
2. Live Free or Die, John Ringo. To free the world from the grip of the Horvath race, it is going to take an unlikely hero.
3. Childhood's End, Arthur C. Clarke. The Overlords come to Earth, destroying the things that cause suffering amongst humanity. However it comes at a price.
4. The Puppet Masters, Robert A. Heinlein. Evil, slug-like aliens land on Earth and attach themselves to people.
5. The Alien Years, Robert Silverberg. Aliens plunge humans into a new Dark Age without electricity. They allow the humans to live but disarm them as the dominant species.
6. Orphanage, Robert Buettner. Under siege, humanity gambles on one desperate counterstrike against an alien force.
7. Invasion of the Body Snatchers, Jack Finney. Dr. Miles Bennell discovers his neighbors, friends, and lover's minds and bodies are being taken over by an alien life-form.
8. The White Mountains, John Christopher. The world is ruled by Tripods. All children are “capped” at the age of 14 when they have a metal cap grafted to their heads and they come under control of the tripods.
9. The Invasion, Katherine Applegate. Jake, Rachel, Cassie, Tobias, and Marco have the power to morph into any animal they choose and must use their power to fight a powerful evil.
10. Way of the Pilgrim, Gordon R. Dickson. Shane Evert sees himself as an ordinary person, but becomes key in the war to defeat the Aalaag, aliens who have conquered Earth.