Building a Settler
Settlers are used primarily to found new cities. Move the settler to the desired city site, and, when the settler is active in that square, press B.
Joining a City
A settler may also be used to increase the population of an existing city; when the settler is active on a city, click the "Join City" button. The settler disappears and the city's population gains two citizens.
When cities grew to a size where the resources were insufficient to adequately insure a decent standard of living for the populace, adventurous groups of citizens set out on their own in search of a place to build a new city. Once a suitable site was found, the settlers would build their new homes, and develop the land surrounding the city. Eventually the whole process repeated, and the new city would send out settlers of its own. This process allowed civilizations to grow throughout history, from the empires of the ancient world to the discovery and settlement of the New World.