Earth, besides the literal meaning of the word refers to our own realm of reality. The reason for adding it here as an article is primarily to make a distinction between Sanctuary and Earth, if another article needs to make comparisons between the two.
Earth is a planet able to support life in a solar system located in one of the spiral arms of a galaxy called the Milky Way. It's about 4.5 billion years old, and has supported intelligent life only a microscopic portion of that time. This life evolved from one-cell organisms called bacteria into a variety of animals, insects and plants.
There's no proven supernatural energies, entities, deities or planes or reality and this is the main difference between Earth and Sanctuary.
Earth and Sanctuary
The main intelligent life and cultures of Earth are very similar to those of Sanctuary, however, and even though that might suggest some sort of infuse of life on Sanctuary from Earth, we know the two worlds share no connection. The humans of Sanctuary are direct ascendants of the nephalem, and not created through an evolutionary process starting with single cell life like on Earth.
Although it is technically possible that the current Angels and Demons are in fact decendant's from Earth in a distant future, and the fuse of these entities genetic material created the nephalem which in turn were turned into humans, it's a far stretch of imagination.
Development (of Diablo)
Blizzard has used the cultures of Earth to inspire them for the cultures of Sanctuary. Most prominent is of course the traditional medieval Western cultures of Westmarch and Khanduras, the Middle East culture of Kehjistan and Lut Gholein, Eastern Asian mix on the island of Xiansai, Viking/Gaelic culture with the Barbarians, Earth jungle tribes are just like the Torajan jungles and the ancient Greek culture portrayed in the Skovos Isles and the Amazon tribes.
Many of these cultures even share names and language with Earth equivalences. The names of many Barbarian and Druidic texts are basically Irish, Abd al-Hazir has a real Arabic meaning, and many similar examples are available.
Mostly, the names and words are made up to sound like they come from said culture, however, and the fact they are sometimes real words could be a coincidence.