Basic science building of the Ancient Era.
The Library increases the science output of the city, speeding up a civilization's acquisition of new technology. It is the first science building, so it's a must (all other buildings from the science chain require it), and its effect is greater the greater the population of the cities it's built in.
You need a Library in every city in order to unlock the National College.
Note that the Great Library wonder provides a free Library. If you plan on constructing this wonder in one of your cities, don't bother building a Library there!
A library is a building housing a collection of books, scrolls, and other written material. Historically, books have been fairly expensive, often beyond the reach of a civilization's poorer classes. The library allows the poor access to the same knowledge as the rich, greatly improving their chances of success and advancement.