What is a tangentially fired boiler?

A tangentially fired boiler has between 4 and 8 burners, placed in the corners of the boiler. These are sometimes angled vertically to maximize impingement of the flames. They are fired into the center of the boiler, not directly into the center, but at the wall to adjacent to them. This causes the flames to swirl inside of the boiler, causing an even distribution of flame throughout the boiler. This is shown pretty well here: http://www.alstom.com/power/coal-oil/utility-boilers/firing-systems

This website also has a lot more detail than previously provided.