Contrary to what is implied by a previous review, this book isn't a clarion-call to arms against globalisation and free-trade agreements, if you're looking for that, check out George Monbiot and similar writers. Having said that, there is a call for developed countries (through enlightened self-interest) to allow developing nations to erect tariff barriers while they develop - something that the early USA used to shield it's fledgeling industries from European manufactures... to the great benefit of both competing parties in the long run.
An example of a key concept: import replacing, whereby a city imports (say) bicycles, leading to development of its own bicycle repair industry, thus gaining skills and component manufacturing in this area, and so going on to become a bicycle exporter in its own turn. In the meantime, the previous bicycle exporting city will (if it has a healthy, import-replacing economy) also have moved on to other, new activities by replacing an import of its own.
Jacobs uses concepts like this to explain, convincingly, facts as diverse as the poverty of the Tennessee Valley area at the time of the famous (and ineffective, according to Jacobs) TVA project, the rise of the Asian tiger economies, why many cities stagnate, and the apparently inevitable decline of great military/imperial powers.
If you've read Jane Jacobs' earlier book on this topic (The Economy of Cities), her main theses on city economies will be somewhat familiar, but this is possibly more accessible and up to date; if you're only going to read one of Jacobs' books, then make it this one. You might then, like me, go back and lay your hands on everything that she has written that is still in print. For anyone who has an interest in economics, she really is that good.