Extraterrestrial (or extra-terrestrial) may refer to any object or being beyond (extra-) the planet Earth (-terrestrial). Aliens are a general term for extraterrestrial lifeforms.
Contact with aliens
Legends and myths dating back many centuries have been written about superior beings that visit Earth from the heavens. In the 20th century these legends came to be interpreted by some as stories of alien visitors, possibly even as ancient ancestors to the human species. (Calling the Twilight Zone)

Aliens in Earth pre-history
Alien biology

Varieties of aliens
Such changes can be witnessed when encountering the Twilight Zone. For example, aliens from Venus have been observed with a third eye, while Martians have been seen with three arms. Like on Earth, living beings from the same planet did not all share the same physiology. For example, while some humanoid aliens from Mars did have three arms, others had only two. Some Martians had one head, while others had two heads. The same was true of Venusians, some being very small in stature while others were a height typical to humans and not all possessed three eyes. (TZ1: "Will the Real Martian Please Stand Up?", "Mr Dingle, the Strong")