Originally the term is empty of content beyond 'not Greek'. The Greeks encountered scores of different foreign cultures (Egyptian, Persian, Phoenician, Etruscan, Roman, Carthaginian, Kurdish, Basque and so on) which had no characteristics in common with each other. (Plato Statesman 262de rejects the Greek-barbarian dichotomy as a logical absurdity on just such grounds: dividing the world into Greeks and non-Greeks tells you nothing about the second group.) It is not the case that Greeks automatically despised all alien cultures. They were aware of the greater antiquity of the much more developed civilisations of Egypt and Mesopotamia, from whom they borrowed extensively.
In Homer the term appears only once (Iliad 2.867), in the form barbarophonos ("of incomprehensible speech"), used of the Carians fighting for the Trojans. Notably the Trojans themselves, who despite bearing Hellenized names in the Homeric telling are emphatically not Greek, are not called barbaroi. In general the concept of barbaros does not figure largely in archaic literature (before 5th cenury BC).
A change occurred in the connotations of the word after the Greco-Persian Wars in the first half of the 5th century BC. Here a hasty coalition of Greeks defeated a vast empire. Indeed in the Greek of this period 'barbarian' is often used expressly to mean Persian. In the wake of this victory they began to see themselves as superior militarily and politically. A stereotype developed in which hardy Greeks live as free men in city-states where politics are a communal possession, whereas among the womanish barbarians everyone beneath the Great King is no better than his slave. This marks the birth of the cultural view termed "Orientalism".
A parallel factor was the growth of Chattel slavery especially at Athens. Although enslavement of Greeks for non-payment of Debt continued in most Greek states, it was banned at Athens under Solon in the early 6th century BC. Under the Athenian democracy established ca 508 BC slavery came to be used on a scale never before seen among the Greeks. Massive concentrations of slaves were worked under especially brutal conditions in the silver mines at Laureion (a major find there in 483 BC), while skilled slave craftsmen producing manufactured goods in small factories and workshops becoming increasingly common. Furthermore, all but the poorest of Athenian households came to have slaves to supplement the work of their free members. Slaves were no longer the preserve of the rich. Overwhelmingly, they were "barbarian" in origin, drawn especially from lands around the Black Sea such as Thrace and the Tauric Chersonese (Crimea), while from Asia Minor came above all Lydians, Phrygians and Carians. It is hard not to despise the people you are keeping as your slaves, even essential: in the intellectual justification of slavery (Aristotle Politics 1.2-7; 3.14), barbarians are slaves by nature. From this period words like barbarophonos cited above from Homer began to be used not only of the sound of a foreign language but of foreigners speaking Greek improperly. In Greek the notions of language and reason are easily confused in the word logos, so speaking poorly was easily conflated with being stupid-an association not of course limited to ancient Greeks.
Barbarian is used in its Hellenic sense by Paul in the New Testament (Romans 1:14) to describe non-Greeks, and to describe one who merely speaks a different language (1 Corinthians 14:11). The word is not used in these scriptures in the modern sense of "Savage".
The Chinese (Han Chinese) of the Chinese Empire regarded the Xiongnu, Tatars, Turks, Mongols, Jurchen, Manchu, and Europeans as barbaric. The Chinese used different terms for barbarians from different directions of the compass. Those in the east were called Dongyi (东夷), those in the west were called Xirong (西戎), those in the south were called Nanman (南蛮), and those in the north were called Beidi (北狄).
The Japanese adopted the Chinese usage. When Europeans came to Japan, they were called nanban (南蛮), literally Barbarians from the South, because the Portuguese ships appeared to sail from the South.
Converted barbarians have historically proved sometimes the staunchest supporters of the more developed culture they have recently subverted. Historic examples are the Lombards and the Manchu. "The best Romans," wrote Henry James, "are often northern barbarians."
Even today, barbarian is used to mean someone violent, primitive, uncouth or uncivilized in general, in very much the same disapproving and superior sense that Edward Gibbon used the term in Decline and Fall of the Roman Empire, which recounts how "the Roman world was overwhelmed by a deluge of Barbarians" a usage epitomized in Gibbon's Book I, chapter 38:
Compare the modern usage of Philistine.
The culture of the Nomad is not to be confused with the barbarian. "Culture" should not simply connote "civilization": rich, deep authentic human Culture exists even without civilization, as the German writers of the early Romantic generation first defined the opposing terms, though they used them as polarities in a way that a modern writer might not.
The nomad subsists on the products of his flocks, and follows their needs. The nomad may barter for necessities, like metalwork, but does not depend on civilization for plunder, as the barbarian does.
In Fantasy novels and role-playing games, barbarians (or berserkers) are still depicted as brave uncivilized warriors, often able to attack with a crazed fury. Conan is simply best known of the type.