A dedication is a note at the beginning saying for whom the book was written. Think of dedications as being in honor of someone (or something). The dedication doesn’t have to be specifically related to the book; think of the dedication as a gift and the book is just the means of conveying it.
Who do you dedicate the book to? Think about who inspired you, who is important in your life, or who you’d like to make happy. You can dedicate it to a single person, to several people, a group of people, an organization, or even an idea. If there are multiple authors on a book, each author gets a dedication.
Dedications can be a great way of showing that you’ve made it to a point: for example, a dear friend of mine wrote a book about something I kept trying to tell him about years before. He had a dedication to me that said “And to my old friend, John, who kept trying to tell me about this when we were in high school–I got it, John, I finally got it!”
The dedication can be something as simple as “To [name of significant other]” or something a good deal longer. I dedicated an early book to my father, the history professor, and had several sentences of text about why I am so pleased to be his son.
Dedications (and acknowledgments) can be fun. One of my favorites was a friend’s dedication to his wife: “To Jill, without whom I would still be impossible.” It should probably also be pointed out that punctuation and structure are incredibly important in dedications. The best example of this was the following nugget resulting from the omission of a serial comma: “To my parents, God and Ayn Rand.”
I’ve dedicated books for wedding gifts, to friends on the occasion of the birth of their first child, to my high school graduating class, and in honor of the retirement of someone who was instrumental in getting me started in the writing biz. I’ve dedicated two books to significant others, but, oddly enough, both relationships broke up not long after. I am very keen on the woman I’m now married to, so I’m not planning on dedicating a book to her, ever.