Aliens is a term used to refer to all lifeforms, usually sentient, that are not from Planet Earth. Since World War II, study and fear of aliens, specially of a potentially successful alien invasion, had grow over human population.
It's possible that alien life had preceded the creation of Earth and may have been evolved in the process of the appearance of life on Earth. It is also possible that they are responsible for the existence of humankind and that magical creatures possess their powers (by the fact that Martians seem to know that Wizards exist, and may even they have magic of their own).
The appearance of aliens is different from species. Some, like Martians are green-skined and short, others can resemble humans.
Much of aliens culture is unknown, but some aspect are usually parodies, such as the human-looking aliens addiction to milk-shake or the fact that Martians dislike when humans use their martian-like masks, seeing it as prejudice.
Relation with magic and Earth
In the episode Disenchanted Evening, it seems that Martians acknowledge Wizards and magic actually exists. They seem to be disgusted by the masks that resemble Martians, believing it to be prejudice. This might mean that aliens actually possess magic or at sometime visited Earth when magic was still strong.
Humans believe that alien civilizations pretend to invade the Earth. Justin Russo is actually quite obsessed by aliens, to the point that he invented an alien language that he believes to actually be real. Also his obsession could have resulted in magic to be exposed beyond repair if the exposure had not been a hoax.
List of Appearence
Disenchanted Evening (first appearance)
Wizards Exposed (mentioned)
Alex Tells the world (mentioned)