Abraham Lincoln, the 16th US President is famous for two main reasons- leading the Union to victory in the Civil War, thereby reunifying the nation, and ending slavery.
By the time Lincoln ascended to the presidency in 1860, the South's intention to secede was clear. But Lincoln, who considered unity the highest priority, attempted to appease the South by promising not to go after slavery in regions where it was allowed. He spoke of the Southerners as brethren and did not adopt the hostile stance that many Northerners did towards the Southern states.
However, following the South's attack on Fort Sumter, the Civil War began in earnest.
Some lesser known details about Lincoln's presidency during the war include: his suspension of habeas corpus- the right to a fair trial, and his original plan for the slaves- sending them to Haiti and Panama. While he believed in the wrongness of slavery, he did not foresee a country where blacks and whites lived together in harmony. However, in 1863, in the middle of the war, Lincoln issued the Emancipation Proclamation. The proclamation had several uses- it gave the weary North, which had been expecting a quick victory, a renewed cause to fight for. It ended slavery, which Lincoln and much of the North loathed. And many former slaves, upon fleeing their masters, ended up taking arms with the Union.
Lincoln is also well-known for his Gettysburg Address, in which he gave a 2 minute speech addressing the Union and giving them a new hope to fight in honor of the men slain at the Battle of Gettysburg.
Lincoln was re-elected in 1864, and the South's defeat was clearly imminent. Lincoln had sympathetic plans for Reconstruction- he planned to allow the re-institution of statehood (and all the rights granted therein) should 10% of the state's population vote for it. Many radical Republicans wished for a harsher and more rigorous administration against the traitorous South.
But ultimately, Lincoln did not live to see through Reconstruction. He died on Good Friday, April 14th, 1865, just days after the end of the war, at Ford's Theater. He was shot by actor John Wilkes Booth while viewing the play Our American Cousin. His Vice President, Southern-sympathizer Andrew Johnson, chosen mainly to appease the South, ascended to the Presidency.
Lincoln today is consistently ranked amongst the top 5 or even top 3 Presidents.
Sources: Wikipedia, White House.gov, Miller Center