Explain how the Civil War changed the nation.

The Civil War dramatically changed the nation.  Businesses who received war contracts for mass-produced goods turned their skills to the civilian sector after the war.  The rail system, which brought troops and material to both armies, was now used to ship raw materials all over the country.  Rail production grew considerably after the war, most notably with the completion of the Transcontinental Railroad in 1869.  
Socially, the North and South did not fully trust each other, and the war left many with hard feelings and distrust.  It was not until the Spanish-American War in 1898 that Northern and Southern congressmen began get along.  The Civil War ended slavery in the United States, and soon afterwards, African Americans received American citizenship and voting rights.  The war also killed over six hundred thousand Americans, and the federal government got involved in the creation of national cemeteries, most notably at Arlington.  The North also provided pensions for Union veterans; these would continue to be a large chunk of the federal budget for years to come.  Union veterans also tended to vote as a bloc—many African Americans did as well—making the Republican Party the major force in American politics for the rest of the century.  
The most important legacy of the Civil War was that secession was never considered seriously as an answer to the nation's political problems again.