An eBook is any file which can be read on a computer, whether it be desktop, laptop, tablet or smartphone. The most common formats are PDF, EPUB and Mobi (Kindle) formats.
Of the three, PDF is by far the easiest to create and will totally maintain the look of a printed piece. It will be viewable on a tablet or smartphone with Adobe Reader or third party PDF readers. It could be uploaded to their website but you might need to export it with lower resolution images for quicker download.
You could include simple hyperlinks, but if you're expecting it to work on tablets and smartphone, I would stay away from fancier interactivity.
I would recommend to use a later version of InDesign than CS5 to export an epub because this functionality has been changed the most in the newer versions beginning with CS5.5 up to CC now.
You can create from the content of the same indd a pdf for printing and an epub, from CS6 you can use an alternative layout to make some changes to improve the reading experience on electronic devices or you can make a new indd based on the old one.
From CS5.5 upward you can add a tag in paragraph styles which will be relevant in all html export types as epub is one of them.
As an epub is a flair of html you have to live with that the epub might look different on different epub readers or computers, same with embedding fonts. Not every font can be embedded not every program which will be able to read the epub is able to use your fonts. Formatting is done with CSS. InDesign will create automatically a CSS based on the formating in you file and your tagging but you can choose to use any other CSS with the correct classes of your file.
At the moment I would recommend to make some additional editing in the epub after exporting it to InDesign. But things are going forward very fast here.