The original Latin is "Gallia est omnis divisa in partes tres, quarum unam incolunt Belgae, aliam Aquitani, tertiam qui ipsorum lingua Celtae, nostra Galli appellantur."
This translates to:
"All Gaul is divided into three parts, one of which the Belgae inhabit, another the Aquitani, the third those who in their own language are called Celts, in our language Gauls."
Caesar uses this line to introduce his description of the geography and people of Gaul, which he conquered in a series of campaigns from 58 to 50 BC.
The phrase has become a famous example of classical Latin and a literary device used to introduce a topic or provide a clear structure. It has also been used in popular culture, often humorously, to refer to dividing things into three parts.