There are two type of window replacements. The easiest and least expensive repair is to replace the window inlet. This leaves the window sill and frame alone and just replaces the window itself. Your repairman will come out and measure the windows, making sure the frame and sills are still in good condition, and order the right sized windows to be installed. However, if the frames are damaged or too off square, the entire window, frame and all, will have to be replaced. This involves removing both the inside and outside trim and repairing any damage done while removing the frame.
Read the full article here: The Basics of Replacement Windows