Short answers, pulled from the story.
A city is a permanent and densely populated human settlement with administratively defined boundaries whose members work primarily on non-agricultural tasks. There is no universally agreed definition of the lower boundary for a city's size, and the term means different things around the world.
Most archaeologists consider Uruk, in ancient Iraq, from the mid-fourth millennium BC, to be the first true city. It innovated many characteristics that later cities followed, and its name is attributed to the Uruk period. Jericho and Çatalhöyük, dated to the eighth millennium BC, are among the earliest proto-cities.
More than half of the world population now lives in cities. The Population Division of the United Nations Department of Economic and Social Affairs reported in 2014 that for the first time, more than half of humanity lived in cities. In 1900, only 15% of the world's population did.
Cities create their own microclimates because concrete, asphalt, and other artificial surfaces heat up in sunlight. The temperature in New York City exceeds nearby rural temperatures by an average of 2 to 3 degrees Celsius, with 5 to 10 degree differences recorded at times. Aerial particulates increase rainfall by 5 to 10%.
A global city is a prominent centre of trade, banking, finance, innovation, and markets. Saskia Sassen used the term in her 1991 work, The Global City: New York, London, Tokyo, to refer to a city's power, status, and cosmopolitanism rather than its size. Critics such as Paul James call the term reductive and skewed.
Dense cities hold the potential for a smaller ecological footprint per inhabitant than sparsely populated areas, due to efficient transportation and smaller land consumption. For this reason compact cities are often called a crucial element in fighting climate change, and the international community has prioritized investment through Sustainable Development Goal 11.