Defining Features:

Aliens that look like reptiles or insects; aliens that look exactly like humans, but turn out to be reptiles or insects in disguise; slime, goo and/or sludge; an unstoppable powerful extraterrestrial force whose scientific knowledge and military strength surpasses ours thousands of times over; the human race succeeding through sheer pluck, moxie, or dumb luck; the aliens either having a single glaring weakness, soft spot or incredibly imbecilic understanding of their own fundamental biology; an important lesson about the resilience of the human spirit.

Origins:

H.G. Wells' War of the Worlds first brought violent alien invasion into the minds and nightmares of children around the world, along with the guarantee that any such aliens would have some incredibly stupid weakness. Despite crashing down on Earth with super-robo octopi that shot heat lasers and just generally wrecked all our shit, the poor bastards were eventually done in by the common cold.

Wells set the stage for future lame alien weaknesses like water in Signs and the screaming vacuum of space in Alien.

See Also:

Battlefield Earth, V, The Arrival, Ender's Game.

Why it Will Never Happen:

Because so far the only aliens technologically advanced enough to visit Earth seem primarily interested in abducting hicks and probing anuses. These poor creatures cross the vast emptiness of space only to crash their saucers in New Mexico and have no technology to avoid detection by farm folk with disposable cameras. The chances for a successful full-scale invasion do not appear to be strong.

4 An Invasion by Friendly Aliens

Defining Features:

Aliens that look either exactly like humans or like adorable humanoid stuffed animals; mankind foolishly squandering what could have been the key to curing all disease and suffering; enormous arrays of multicolored lights; someone achieving weightlessness while looking wondrously awed; someone touching an alien for the first time while looking wondrously awed; long shots of people looking up at the night sky; an important lesson about the mistrusting and primitive nature of the human spirit.

Origins:

Alien invaders don't always want to kill us, but when they don't, we usually prove why they should. It all started with the 1951 classic The Day The Earth Stood Still, when an alien named Klaatu and his robot friend Gort came to Earth to save us all from ourselves. They were shot within 10 seconds of landing, imprisoned, escaped, got turned in by a filthy human for the reward, then got shot again ... to death.

But, it's even worse than that: Klaatu wasn't any ordinary interplanetary goodwill ambassador. If you put the pieces of the puzzle together-a mysterious visitor from the heavens, suffering for our transgressions, gets turned in by someone he trusts and is ultimately sacrificed-it becomes fairly clear that we killed Jesus. Again! Way to go, humanity. Real classy.

In humanity's defense, we did treat the Close Encounters of the Third Kind aliens a little better. Though if that film had run another 30 minutes, maybe we'd have seen CIA interrogators hooking electrodes to the aliens' genitals and demanding to know what their ships use for fuel and how we can get our hands on it.

See Also:

Contact, Stranger in a Strange Land, 2001: A Space Odyssey (or at least the third hour, with, you know, the aliens).

Why it Will Never Happen:

It's an irrefutable scientific fact that a species cannot evolve to dominate its planet unless it is made up of merciless killing machines. Any civilization with access to the resources necessary to reach us, has, by definition, gained that access by slaughtering its biological competitors. If they turn up here tomorrow, it's only because they've found out, say, that our ground-up spleens are an afrodisiac for their women.