The Aliens were the invaders who attacked Earth in Steven Spielberg's War of the Worlds. Their appearance is vastly similar to the Tripods they brought to invade Earth. Usually, in the War of the Worlds tales, Aliens from Mars take invade Earth, but in the 2005 remake, the aliens are not from Mars. They are from an unknown planet, and nothing is known about how it looks like or where it is located. Steven Spielberg made this change because of recent NASA missions to Mars, proving that their was no signs of large life on Mars (that does not mean there is none, in light of the recent discovery of water on the planet's surface). He changed the aliens' homeplanet to an unknown location, and mentioned in the movie's trailer, the aliens had been planning the invasion of Earth for millions and millions of years, which could mean the planet is in another remote Solar System located somewere in the Milky way, which is a long way from Earth.
Also, in the original story, the Martians had many devices to attack mankind, such as the Flying-Machine, The Fighting Machine, and The Handling-Machine. However, in this adaptation the only machine they brought was the Fighting Machine known as the Tripod. Still, the story has vast similarities with the other adaptations, and the tripods have some of the same powers as the original (Heat Ray (which can also zap any living thing into dust in this adaptation, Red Weed Planters, Tentacles, and Prison Chambers for abducted humans. The tripods lacked black smoke in this adaptation, however).
One more difference, is that instead of coming in Cylinders, they come inside of the lightning. Also, instead of attacking first with just a heat ray, they use a tripod.
The aliens themselves have two minute arms hanging to their chest, two long, skinny arms with three fingers on each hand, and one leg. The heads are shaped in triangles, which resembles their Tripods. Their eyes are black, and they have two sets of sharp teeth. They also appear to be highly intelligent.