Definition from Wiki:
A bookplate, also known as ex-librīs [Latin, "from the books of..."], is usually a small print or decorative label pasted into a book, often on the inside front cover, to indicate its owner. Simple typographical bookplates are termed 'booklabels'.
Bookplates typically bear a name, motto, device, coat-of-arms, crest, badge, or any motif that relates to the owner of the book, or is requested by him from the artist or designer. The name of the owner usually follows an inscription such as "from the books of . . . " or "from the library of . . . ", or in Latin, ex libris .... Bookplates are important evidence for the provenance of books.
Read more at http://en.wikipedia.org/wiki/Bookplate