Urbanisation can have many effects. National BR and TFR goes down as children become economic liabilities in cities compared to farms where they're needed to tend to land. Rural-Urban movment creates demographic effects in the rural areas, the dependency ratio increases (an hourgalss population pryamid is left by the young workers leaving). In China there are 70 million children that have been 'left behind' by their parents who've gone to the cities, and they suffer socially, they lack formal education from often illetrate grandparents.
Economic development is undobutly a major driver of urbanisation, why do people move to cities? To get a better job or more money (combined with lack of rural jobs due to mechanisation of farming etc), traditionally, urbanisation coincides with industrilaisation, Manchesters population doubled from 1801-20 as people flocked to work in the textile industry. Economic effects may be the remittances sent back to rural area, Chinese hyperurbanisation in its SE provinces (the SEZs), combined with other factors has undoubtly led to its 9% pa growth from the 90's onwards.
Urbanisation has environmental impacts, squeezing lots of people suddenly into an area can create polloutio, make diseases transmit very quickly and easily, urbanisation and urban sprawl, suddenly lateral expansion engulfs countryside.