Manifest Destiny affected the lives of many different groups within the U.S.
Lower and middle class farmers were affected by the large tracts of land opened by territorial acquisitions and conquests. As the U.S. expanded its reach west, new lands were opened up for settlement. Farmers, shopkeepers, trappers and many other groups began settling in these lands, starting new lives and expanding the U.S. economy.
Other countries were affected by Manifest Destiny as well. Great Britain, France, Mexico, Russia and a number of other nations all saw their borders change or their overseas colonies shrink and grow thanks to American expansionism. Sometimes it was mutually beneficial, such as in the case of France and the Louisiana Purchase. Sometimes it wasn’t, such as in the cases of Spain’s loss of Florida or Mexico’s loss of its northern provinces.
The group most affected however were Native Americans. As settlers moved west, natives were pushed off their tribal lands or forced into losing battles with the American military. Sometimes they were pushed on to reservations, like in the case of the Navajo or Sioux. In other cases they were totally wiped out, like the Modoc of northern California. Diseases, Christian missionaries and alcohol also played a role in the destruction of the native way of life.
Born in the 1840s, Manifest Destiny is the idea that America was destined by God to colonize the whole of North America. This idea was important in the formation of American identity during this period and impacted a significant proportion of the population.
Christian missionaries, for example, were inspired by Manifest Destiny because it provided them with the opportunity to spread the Christian message. In their eyes, the Native Americans were uncivilized heathens who ought to be converted to Christianity, and they experienced widespread prejudice and discrimination, including the loss of their lands, as a result.
Manifest Destiny also had an impact on writers of this period. For instance, the poet Walt Whitman (1819-1892) wrote poems which romanticized the idea of westward expansion. One example is "Pioneers! O Pioneers!" which emphasizes the bravery and dedication of those who headed westward and settled the land. For more examples, see the reference link provided.