On the Digital Photography School blog, Gavin Phillips writes,
What is High Dynamic Range Imaging? (HDR)
HDR is when you take 3-5 or 7 photos at different exposure settings, and then merge them into a single image using speciality software. What you get are beautiful photos with incredible detail, controlled lighting and accurate colour. You cannot reproduce an HDR image manipulating a single JPG or RAW image in Photoshop.