A booktalk in the broadest terms is what is told or spoken to convince someone to read a book. Booktalks are usually conducted in a classroom setting for students. However, booktalks can be performed outside a school setting and with a variety of age groups as well.
A booktalk is not a book review or a book report. The booktalker shares a little bit about the setting, the characters, and the major conflict without "giving away" the ending. Booktalks make listeners care enough about the book to want to read it. A long booktalk is usually about five to seven minutes long and a short booktalk is generally thirty seconds to two minutes long. So short and sweet is the key and is also what makes booktalks engaging and fun for kids.
So the next question has to be....how do I give a book talk?
Look up some examples of great book talks with a site developed by Nancy Keane.
This site provides many examples of booktalks and may help you choose books to read aloud to your young readers as well as help you give short booktalks to help them choose a new and intersting book for themselves.