Your civilian units work on the Terrain Map. 

Engineers are used to connect the resources to your transport network so the resources may be used to feed  workers and supply  industries. If a terrain tile is within one tile of a connected road,  port, or the capital city, the resources in that tile are available to the transport network. 

Explorers look for mineral resources and reveal hidden tiles in the New World.
Builders increase the number of resources produced by a terrain tile.
Merchants purchase land from Tribes and Neutral States.
Spies investigate military strength and steal technology.
Railroad workers convert roads to railroads.