What is the difference between First Edition and a First Printing?
What is the difference between a signed and an inscribed book?
Is an inscribed book worth more than a signed book?

Questions on the Preservation of Books.
Your Old Books (RBMS Publications Committee (Revision 2005.2) 02/07/2006)

What is the difference between First Edition and a First Printing?
According to the Antiquarian Booksellers’ Association of America a First Edition is defined as follows (italics mine):

“All of the copies printed from the first setting of type; can include multiple printings if all are from the same setting of type. Every printed book has a first edition, many never have later editions. A later edition would have substantial changes in the printing plates or type such as the addition of a new preface or new chapter or major changes throughout the text and often is printed from a complete resetting of the type. When book collectors use the term first edition, they are usually referring to the first printing and if there are different states or issues, the earliest of those.”

The last line is the relevant one for us collectors. Really, when collectors talk about First Edition they are really talking about the first print run of the First Edition.

What is the difference between a signed and an inscribed book?
The term “inscribed” indicates that the author signed the book to the person noted by name in the inscription.  A signature without any other inscription is sometimes called “blank signed”.

Is an inscribed book worth more than a signed book?
Generally not but there is no consensus among booksellers.  If the person to whom the book is inscribed is of note (another author or famous personality) then the book may judged to have added value.