For the general description of XP bonus, see Ghost (bonus).
Ghost is the name for both an achievement and an XP bonus in Deus Ex: Human Revolution. The Ghost achievement is gained simultaneously with the first Ghost bonus. There are plenty of opportunities to earn it, considering that there are 29 Ghost bonuses available in the game (30 with additional mission from Explosive Mission Pack DLC).
Although the Ghost bonuses vary greatly in difficulty depending on mission, the achievement is one of the easiest in the game: all you need to do is to receive the Ghost bonus for completing "Enter the main building" objective, which is a part of Securing Sarif's Manufacturing Plant mission.
After you arrive at Milwaukee Junction, head down to the ground level (Pritchard will contact you and suggest attempting a stealthy entrance). Around the corner will be a guard with his back to you and talking on the phone. Perform a takedown to take him out (lethal or non-lethal, as you prefer). Along the wall on the right is a crate sitting in front of a forklift; take the crate and place it against the steel container to climb up onto the container. Keep climbing up until you reach the roof. You'll notice the path ahead is blocked by exposed electrical cables, but there is a vent hidden behind a small crate to the right; move the crate and crawl through the vent. When you emerge, use the switchbox in front of you to turn off the electricity, then continue along the roof until you find another vent leading inside the building. You'll gain the achievement as soon as you go inside.