Most people move from rural to urban areas for economic reasons. Economic factors range from jobs to cheap goods and services. Because paying jobs are limited in rural areas, most rural dwellers are forced to seek jobs in urban areas. Most jobs in rural areas will usually be for self or family sustenance such as subsistence farming which only bring food but rarely money. Money is needed for health services, education and buying goods, thus rural dwellers are forced to migrate to urban areas in search of paying jobs.
The Green Revolution which began in the 1950s brought about a change in agriculture. New technology through the use of machines replaced rural people who already worked in farms. Thus with no jobs, people were forced to migrate to urban areas to look for jobs. The Green Revolution had a major impact concerning employment in countries such as India and China. Many Indians were forced to migrate to cities which led to the eventual flooding of people in urban areas.
Limited schools in rural areas force people to migrate to urban areas where there's quality education although majority fail to afford this education.
Again, health centers are very few in rural areas, people will have to walk tiresome journeys to get to the nearest health center and some barely make it alive. Even if people make it, the clinics will barely have quality health facilities and adequate medicine.This forces people to migrate urban where there's abundant health centers and medicine.
Most rural dwellers settle near hazardous areas in search of minerals and greener pastures. As such, when phenomenons such as volcanoes or earthquakes occur these people are displaced and have nowhere else to go but urban areas. When Mt Pinatubo (Philippines) erupted in 1991 displaced people were moved to the city Angeles.
Rural dwellers may be forced to migrate urban due to political issues such as wars or terrorism.
The above factors mostly force rural people to move urban. However, pull factors can also cause rural-urban migration.
NB: Factors such as job or education maybe forced or pulled depending on the perspective (area) you're viewing on. If you explain schools, for example, in the context of urban areas, you will explain them in good light and hence people are "pulled" from rural to urban areas. But if you explain schools in rural areas, you will often explain them as undesirable, and thus people are "forced/pushed" to urban areas.
Often rural people talk about and admire the "Bright Light" and entertainment of the city which can actually pull them to migrate into the city; they see the city as a paradise.
Rural people who have successfully settled and are enjoying urban life can contact and encourage their friends or relatives remaining in rural areas to also settle in the urban area.
Rural dwellers can visit their friends or relatives in urban areas. Although temporary, some can stay permanently.
The most noticeable impact of rural urban migration is urbanisation which is the increase in urban dwellers. When the city is filled with a huge number of people, many problems arise:
Because some migrates fail to get proper jobs in urban areas, lack of money forces them to build illegal settlements called shanties. These settlements will often be built using poor and cheap materials such as plastics and card boxes.
In addition, these settlements will be built on land not allowed or recommended to be built on; for example in Brazil shanties called favelas are built on mountain slopes and in India shanties can be located around the heart of the city such as Mumbai.
Moreover, these settlements are manifested by poverty and diseases such as cholera due to poor sanitation. Millions of dollars are wasted trying to improve the quality of these settlements.
Lack of jobs forces some migrates to deal in illegal activities such as theft, drugs and prostitution.
The increase in people hence pollutive activities result in the development of urban heat islands. This results in high temperatures, abnormal rainfalls and the weather in general.
However, rural areas also suffer from brain drain as they're deprived of skill and labour. This reason alone is the reason why rural areas, especially in LEDCs, are still backwards. If services, jobs, schools and growth points are also invested more to rural areas, rural-urban migration may decrease and rural areas develop.