The answer to that question is, it depends. There are now two types of e-book formats. The first is what we call the basic format. This format is based on XML, XHTML, and cascading style sheets (CSS)-terms you're probably familiar with that are associated with web pages. And this is sort of what an Epub is: a webpage. So you can see your Epub on a small handheld device and the text will flow to accommodate the screen size of the reader you are using. Depending on the capabilities of your eBook reader, you might also be able to change font and font size.
Page structure is not defined in the Epub format so when your book is formatted, everything flows together in one continuous file, images included. If you have a fairly simple layout for your book you might not see much difference in the Epub, but a complex designed book might not lend itself to the Epub format and look totally different. Some content such as page numbers, page breaks and some vector based files are actually discarded when a book is converted to Epub. Because the format is based on several open standards, some things will look different depending on what eBook reader you are using.
The second is called a "fixed layout" format. Although it also relies on XML, XHMTL and cascading style sheets, the fixed layout e-book retains the exact same look as your printed book. This type of e-book may seem the best option on first look but it does have disadvantages. Unlike the basic format where a customer can change font size for better readability, the fixed layout format can only be pinch zoomed to enlarge the text. This may very well render it difficult to read when viewed on smaller devices like IPhones. Created primarily for IOS devices, many apps will have formatting issues when trying to display a fixed layout e-book.