How do aliens differ from humans?
This is a complicated issue. Aliens, (so called) cannot be totally defined by the skinny grey beings with wrap around eyes, seen frequently by earthlings. These beings are synthetic biological beings. They serve as hosts for the consciousness of their creators. The reason they need these hosts is because they come from another dimension and moving from one dimension to another is physically impossible for human beings; and they are human beings!
There is a barrier that divides one dimension from another. In essence, each dimension is like a series of interconnected tubes, within which contain the particles that make up our reality.
Moving from one dimension to another requires that one be able to penetrate the dimensional sub quantum tube walls. Doing this requires a great deal of force. The transfer from one dimension to another puts too much stress on a human body.
To answer your question, what we see are not the aliens themselves. They are hosts to the consciousness of the alien. They themselves are human beings. They look human, they sort of act human, with a few exceptions. They do not have some of the organs we on earth have. For instance, they have no reproductive organs. At one time, they were exactly like us, they were us. They are members of an advanced society from our very ancient past. They became immortal and as a result they are still around today.
In order for these beings to become immortal, they had to leave the earth dimension. Unfortunately they had to make some dramatic physiological changes to themselves in order to move from one dimension to another.
Today they are trying to figure out out how to become completely human and also completely immortal. The answer to this paradox remains till this day, the reason for their constant monitoring of our comings and going's from and to the earth as we incarnate over and over again. They continually monitor our DNA, within which contains a record of everything that happens to us. In essence, they are trying to build a stairway to Heaven.
Divine advanced human beings.com
The nearest ones are single-celled creatures who look very unlike us. They may be on Mars and/or Venus (in the sky).
Somewhere out there, maybe only a few hundred light-years away, there are alien land animals. They are probably bilaterally symmetrical because that's easier for evolution to form. If they have endoskeletons instead of exoskeletons, they will probably walk on four or two legs. If they are large, they will have endoskeletons.
Many land animals will be segmented with exoskeletons, also an easy path for evolution. They will not be too big because of support issues for an exoskeleton.
Aquatic creatures could look like just about anything. Just look at ours. Then, look at Ediacaran fauna. You will see what I mean.
There's not much more to be said because we don't really have much to go on.
I'm not sure how to answer that question, being that as far as I know we've never come in contact with an alien species from another planet, except for maybe alien abductions that we've been told that have happened. For all we know they may actually looks like the creatures from Star Wars, Star Trek and all the other visualizations of what aliens may look like.
If you look at the variety of the different species that exists on Earth alone, basically we're all alien to each other. If you look at all the different colors of humans on the Earth we are aliens from each other color wise, also language wise, also politically, religiously and so on.