The Industrial Revolution transformed Manchester into a textiles manfacturing centre in the late 18th to 19th centuries from cotton sourced in the New World. Things in "Cottonopolis" looked good for the bourgeois, but for the working poor, conditions were dire. Overcrowded and polluted, it gave inspiration to Friedrich Engels and Karl Marx's Communist Manifesto.
There's still a lot of grand Victorian architecture around the city centre, contrasted nicely by new glassy skyscrapers. The city centre is fairly compact and attractions are walkable to each other. I didn't have much time though - 2 days, but the weather co-operated and I had a great time exploring the city centre.