For example Gaul, we all assume, was inhabited by 'Gauls'. In fact there were three different Celtic groups living there. Caesar tells us at the beginning of the Gallic Wars that - 'Gallia est omnis in partis tres, quarum incolunt Belgae, aliam Aquitaini, tertiam, qui ipsorum lingua Celtae, nostra Galli appellantur' i.e. - "All Gaul is divided into three parts, one of which the Belgae inhabit, the Aquitani another, those who in their own language are called Celts, in our Gauls, the third".
So within the umbrella term 'Celtic' there were very different ethnic branches or groups, who however shared similar 'Celtic' cultural traits - language, religion, material culture etc.
Hope that helps

