Is there a program that you can write or create your book as an ebook?

I want to write an ebook. I understand an ebook is a pdf. Is there a program that you write and creat it on. How would you put your pic and graphics in word? Im trying to search but I keep getting programs that will turn my book into a pdf. I want a program that I sit down and write the book . Like you get a webhost and then design a website... You know . Im having a really hard time describing this.