An earthling is someone who lives on the planet Earth. In science fiction books and movies, people are called earthlings to distinguish them from aliens. E.T. was definitely not an earthling.
You're most likely to come across the word earthling in fiction, since we more often refer to ourselves as "people" or "humans," assuming that we are all from Earth. In science fiction, though, some characters come from other planets or galaxies and aren't actually earthlings. Earthling has an Old English root, yrþling, or "plowman," and in 1949 the author Robert Heinlein first used earthling in a science fiction context.