The Indo-Europeans were an ethno-linguistic group of people who according to the Kurgan hypothesis lived around 6000 - 4000 BC in an area corresponding to modern day Ukraine, Russia and Georgia before they invaded Europe, Iran and Northern India and bringing with them new technology, customs, religious beliefs, and Indo-European languages.
The term Aryans refers to Indo-Europeans or certain more specific Indo-European peoples.
A minority view is that the Indo-European languages originated earlier and from the origin of agriculture in the Near East. Genetic studies have found support for both an older immigration from the Near East and a more recent immigration from Northern Eurasia. See the article on Europeans.
Prior to migrating, and the divergence of Indo-European branch languages, the Indo-Europeans were collectively settled in a single territory speaking a single language, reconstructed by modern linguists as Proto-Indo-European.reference required
Trifunctional model and caste system
The Indo-Europeans were often not the only people in the regions they penetrated and they therefore implemented a caste system. This is still important in India.
There were also divisions among the Indo-Europeans. See the article on the Trifunctional model.
Possible causes of the Indo-European expansion
The book Understanding human history: An analysis including the effects of geography and differential evolution argued that "Nor can it be maintained that their remarkable early expansion was due to their possession of superior technology. Quite a few of the peoples they conquered — including the Minoans, the Etruscans, the Elamites, and the Dravidian-speakers of the Indus Valley — had more advanced civilizations than the Indo-European invaders did. It is likely that the some of the early conquests of the Indo-Europeans were due in part to their use of horses; but this could hardly account for their conquest of Crete, Britain, Switzerland, and Scandinavia. Nor would it account for their triumphs over the Egyptians and Babylonians, both of whom had been using horses in warfare for many centuries. The simplest explanation is that the original speakers of PIE possessed, on average, considerably higher intelligence than most of the peoples they defeated".
The same book argued on the achievements of Ancient Greece that "The best explanation for the Greek phenomenon lies in a combination of genetic and geographic factors.... because of the mild climate in the Middle East, and the availability of a large assortment of useful domesticable plants and animals, the inhabitants of the Middle East developed agriculture long before the peoples of northern Europe. The early advent of agriculture and cities in the Middle East enabled them to make major progress during the Neolithic Era and the early historic era, and to get a big jump on the rest of the world in technology and in intellectual matters. In time, the superior genetic endowment of the Europeans would enable them to overcome that head start. However, between European groups, the one most likely to advance first was the one which had the earliest opportunity of learning from the civilizations of the Middle East and Egypt. Because of their geographic location, the Greeks were the first European people to come into contact with those civilizations.... What about the Hittites?.... were a rather small group numerically, and their genes were soon swamped by those of the much larger indigenous population with whom they interbred.... In Greece, there were several waves of invaders, and they entered a mountainous land where the indigenous population had been fairly small. Hence, the gene pool of the classical Greeks was derived mostly from that of the Indo-European invaders, and the average IQ of the resulting population was high."