Short answers, pulled from the story.
Between 46,000 and 51,000 soldiers from both armies were casualties during the three-day Battle of Gettysburg, making it the deadliest battle in American history up to that time. Union casualties were 23,055, while Confederate casualties were documented by Busey and Martin at 23,231 in their 2005 work Regimental Strengths and Losses at Gettysburg.
Lee invaded the North to relieve pressure on war-ravaged Virginia, allow his army to live off Northern farms, and threaten cities including Philadelphia, Baltimore, and Washington. He also hoped the invasion would strengthen the peace movement in the North and potentially reduce pressure on the besieged Confederate garrison at Vicksburg.
Between 10,500 and 12,500 Confederate soldiers advanced three-quarters of a mile across open ground toward Cemetery Ridge on the 3rd of July 1863, under artillery and musket fire. The attack was repelled; nearly half the Confederate attackers did not return to their own lines, and Pickett's division lost about two-thirds of its men.
The Gettysburg Address was a 271-word speech delivered by President Abraham Lincoln on the 19th of November 1863 at the dedication of Soldiers' National Cemetery, which honored fallen Union soldiers. It is widely considered one of the most famous and significant speeches in American history and redefined the purpose of the Civil War.
After Gettysburg, Lee's army conducted no more strategic offensives into Northern territory, shifting to a reactive posture against Ulysses S. Grant's campaigns in 1864 and 1865. The battle also ended any lingering hopes of European recognition of the Confederacy, as Henry Adams reported from London that "all idea of intervention is at an end."
Union Major General George Gordon Meade commanded the Army of the Potomac, having replaced Major General Joseph Hooker just three days before the battle began on the 28th of June 1863. Confederate General Robert E. Lee commanded the Army of Northern Virginia, which included three corps under Lieutenant Generals James Longstreet, Richard S. Ewell, and A.P. Hill.