The main difference between fixed and reflowable formats is how the text is displayed on the screen. In a fixed format, the text is displayed in a predetermined number of columns and pages, and the reader cannot change the way the text is displayed. In a reflowable format, the text can be reformatted to fit the reader's screen or window, and the reader can change the number of columns and pages.
Fixed-format books are generally used for books that will be read on dedicated e-readers, such as the Kindle or the Nook. Reflowable formats are more common for books that will be read on computers or other devices with screens that can be resized.