In Guardians of the Galaxy, we see many members of the Nova Corps. All of them look human. Late in the film, they refer to Star-Lord as a Terran, so they know of Earth, but it's clear they themselves are not considered Terran. Rocket also refers to Star-Lord as a 'humie' multiple times, which presumably is short for human.
Why/how do they look human, if they're not from Earth?
TL;DR - Humans are not limited to Terra, but one from Terra is called a "Terran".
Even within the comics, Humans are not limited to Earth - variations of the race are scattered throughout the universe. The Homo Sapiens race was originally created by the Celestials, a space-faring race easily comparable to gods.
We know that Celestials exist in the MCU, as Knowhere was still shown to be the skull of a Celestial (even though the race wasn't named as such on-screen). Therefore, it stands to reason that the Celestials may still be the creators of the human race, and could have spread their experiment throughout the galaxy. In fact, The Collector himself is one of the oldest beings known to exist and appears very similar to a human in appearance. We know he isn't one due to his nearly-immortal lifespan, but it's quite feasible to theorize that both races have the same origins.
Also, the fact that Peter Jason Quill is stated to be half-human indicates that humans share enough DNA with other races to produce viable offspring. This is further substantiated by the fact that Nova Corps officer Dey is shown to have a half-human daughter with his Xandarian wife.
This question appears in context to many science fiction stories where aliens appear human. One popular answer is called Panspermia; you can read about this in Wikipedia.
In short Panspermia suggests that genetic material is being carried between the stars, perhaps following great cataclysms where an exploding star might sweep away all life on a planet and the remains eventually drift through space to the planets of distant stars.
An alternative Panspermia theory is that some ancient race is deliberately 'seeding' new planets with the genetic materieals required to start up and populate entire eco-systems, complete with all the evolutionary precursors required to evolve human-like species who may be genetically related to one-another; presumably they are also related to the ancient race that made their races possible.
One or more episodes of one or more of the Star Trek series or movies used this theory.
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