Short answers, pulled from the story.
The border states were Delaware, Maryland, Kentucky, and Missouri, four slave states that primarily supported the Union rather than seceding to join the Confederacy. West Virginia was added as a fifth border state in 1863 after breaking away from Confederate Virginia, on the condition that it gradually abolish slavery.
Lincoln reportedly declared, "I think to lose Kentucky is nearly the same as to lose the whole game," because he believed that losing Kentucky would also mean losing Missouri and Maryland. He also said, "I hope to have God on my side, but I must have Kentucky."
About 170,000 border state men, including African Americans, fought in the Union Army, while approximately 86,000 fought for the Confederacy. In Kentucky alone, an estimated 80,000-125,000 served the Union compared to 35,000-40,000 who served the Confederacy.
On the 21st of August 1863, William Clarke Quantrill led a guerrilla raid on Lawrence, Kansas, killing more than 150 civilians. It was carried out in retaliation for Union "Jayhawker" raids against pro-Confederate settlements in Missouri, and it prompted Brigadier General Thomas Ewing to issue General Order No. 11, which forcibly evacuated about 20,000 civilians from four counties near modern-day Kansas City.
Maryland abolished slavery in 1864, Missouri and Tennessee in January 1865, and West Virginia in February 1865. Delaware and Kentucky did not abolish slavery until December 1865, when the Thirteenth Amendment was ratified, and neither state voted to ratify the amendment.
West Virginia was formed from 50 counties of Virginia after western Virginians rejected secession. The statehood bill was passed by Congress and signed by President Lincoln on the 31st of December 1862, and West Virginia was admitted to the Union in 1863. An estimated vote in those 50 counties on Virginia's secession ordinance shows 34,677 against secession and 19,121 in favor.