Located in Washington, D.C., it's a memorial to Abraham Lincoln, America's sixteenth president. Built out of marble in a traditional greek temple style (see The architecture in Washington, DC, is not American). Do you have a penny or a five dollar bill? There's a picture of it on the back. For those of you without such visual aids: it's 80 feet tall, 190 feet wide and 110 feet deep. It's got 36 doric columns (one for each state in 1865) and the names of all 50 states around the top. Inside is a larger-than-life statue of Lincoln himself, sitting in a chair. The walls of the memorial are inscribed with various inspiring and famous things Lincoln said, including the Gettysburg Address and his second inaugural speech.
The memorial is located on the mall. Looking forward, you can see the reflecting pool and, further on, the Washington Monument. Of all the monuments and things in downtown DC, the Lincoln Memorial is most accessible to large gatherings of people, having both a large area for people to sit and a pre-existing stage (the steps to the memorial). Many famous marches, protests, etc., have taken place on the steps of the Lincoln Memorial, most notably Martin Luther King, Jr's "I have a dream" speech.
Some info from the National Parks Service: http://www.nps.gov/linc