The term "Celts" (Kelts) is often thought to refer an ethnic group, usually the Irish and/or Scots. However, it actually refers to a linguistic group of people who speak any of the Celtic languages. Modern Celtic languages left are: Breton, Irish, Cornish, Manx, Welsh and Scots Gaelic.
The ealiest evidence of the Celts was in Hallstatt, Austria. In the early 1800's nearly 1000 graves were excavated in the nearby salt mines. Most of these graves and the artifacts date back to 700BCE. Further evidence suggests that the "Hallstadt period" may date back as far as 1200BCE. Included in the extensive artifacts uncovered were iron weapons and accoutrements indicating that the Celts were arguably the first Europeans to use iron.
In the 1860's at Le Tene Switzerland on the shore of the Neuchatel lake, archaeologists discovered some odd timber piles. They then dredged the area and recovered some 166 swords, 269 spearheads, 29 shields, belt clasps, tools, and iron ignots.
The maximum extent of the Celts is quite vast and covered areas no longer currently thought of as "Celtic" such as: Spain, Galatia (in Asia Minor), Northern Italy, Germany, and France.
Celtic expansion was quite successful early on primarily because of their iron weapon technology and fierceness in battle. In 390BCE they even sacked Rome. And when the Romans complained that the Celts' weights were too heavy to accurately weigh the tribute, the Celtic chief added his sword to balance saying "Woe to the conquered!"
The Celts had little in the way of any central government larger than a modern county since tribes were ruled by a Chieftain. Slowly, one by one, the clans and tribes were over taken and conquered by the Romans from 250BCE to 400CE until the only Celts unconquered by the Roman culture were the Irish, the Scots and the Picts(?). (There is still much debate as to whether the Picts were a member of the Celtic peoples or were a separate distinct language group).
Much of what we know about the Celts comes from Roman and Greek sources. And many of these are either biased, such as the "Gallic War" by Caesar, or from second and third hand sources such as the Greeks Strabo and Diodoros. Although we know the Celts were not ignorant of writing (some Druids were able to read and write Latin and Greek) , they chose not to document their histories or culture. They instead relied on highly trained loremasters/musicians called "Bards" to record and recite their histories, feats, battles and deeds, in song and verse. It wasn't until the Christian monks started transcribing the songs and verse (and then many times with "Christian" modifications), that we had any clue as to the nature of the Celtic mythology, heroes and history. From these sources we can learn something of their Society, Religion and Warriors