The term “Celts“ refers to a great number of different ethnic groups. So we do not speak of one “Celtic people”, at any rate not according to modern ethnic criteria, but rather of tribes within the circle of Celtic culture.
The so-called Celts were organised in tribes – and there were hundreds of them. Although tribal alliances were formed time and again in the course of history, they often waged war among each other.
Presumably, the term Celts was only a collective name used in ancient Mediterranean cultures for geographically widely dispersed tribes north of the Alps with similar languages, religions and customs.
The culture of the Celts is commonly considered nonliterate. It is presumed that they never developed a writing of their own and used letters of other cultures. As the few existing Celtic inscriptions have been almost untranslatable up to now, our knowledge of Celtic culture is derived from Greek and Roman authors – and they were hardly flattering because they saw the Celts and also other tribes or peoples from the point of view of their own culture which they deemed far superior and more civilised.