Much depends on how large or important your city is for how easy or difficult it will be to find information about a city. Encyclopedia Britannica and CREDO Reference -- and others, see the tab for Encyclopedias and Almanacs, above -- are good for basic information for just about all cities. Gale Virtual Reference, below CREDO on the left, is pretty good for bigger cities, and includes a title called Cities of the World, which is really good for big and/or important cities.
The Atlantic Cities Online news portal that delivers reportage on urban and suburban issues. From The Atlantic Magazine.
Population of capital cities and cities of 100,000 and more inhabitants United Nations "presents data on the population of the city proper according to its administrative boundaries and on the urban agglomeration (comprising the city proper plus its contiguous built-up areas)."
The Nationial Geograpnic People and Places - Cities site is useful for basic information and sometimes more detailed articles on cities. (The geographic details on the site are taken from the Atlas of the World, a book to the left.) Fodor's Guides can be great for all kinds of descriptions and details about a city. Find the guide for the country your city is in. (The link is a search of the Diamond library catalog for Temple libraries.) Many of these are in print in Paley Stacks. Fodor's Online can usually provide you with at least a tourist map of downtown.
Most city governments have a department or office dedicated to business or economic development with a web site that will point you to data and statistics on what is best about a city. Search Google for your city and the phrase "economic development" or "business development". For example Chicago and economic development (The office for the city of Philadelphia uses the term "Commerce".)
For history and economic details, see the print Encyclopedia of Urban Cultures, Historical Cities and other sources to the left under Books in Paley Reference.
Proquest Statistical Insight can be useful for tables of statistics for cities-- usually in comparison to other cities. The statistical bureau for the country your city is in can sometimes be useful for finding all kinds of statistics for a city. More developed countries have a site search (and in English) where you can just put in the name of a country.
For more detailed population or city conditions, search Academic Search Premier to see if someone has done a scholarly study of a city. Public health studies are usually pretty good for providing very detailed population details.
Use Proquest Newsstand for recent newspaper articles about cities, particularly if you are searching for "tourism and [citiy X]" or "urban development and [city X]"