What is the difference between theme and motif?

Theme and motif are not the same concepts, but they do work together to convey the writer's message. A theme is the central idea or main message that unifies the entire work. It is the main idea the author is trying to convey through his or her writing. The theme is what the author is trying to say. A work may have one or more themes. All other elements of the work contribute to solidify and communicate the overall theme(s).
Motifs are events, actions, symbols, and ideas that recur often throughout the work. They are images or elements that you will see or hear over and over again throughout the work. Motifs help to emphasize the main theme of the work.
For instance, one of the themes of Charlotte’s Web by E.B. White is friendship, or sacrificial friendship. A spider, Charlotte, attempts to save her pig friend, Wilbur, from certain death and is willing to sacrifice her own life to do so. That is the theme, the main idea or central message, of the book. That is main idea the author is trying to communicate. Motifs of the book include recurring elements that contribute to the theme. Spider webs are a recurring element in Charlotte’s Web and help to emphasize the sacrificial love Charlotte has for Wilbur. Charlotte is using her own means of life (the web, which is used for her sustenance) to preserve Wilbur’s life. Words are another motif of the book. Charlotte repeatedly uses words in her attempt to secure Wilbur’s salvation. So, while theme and motif are distinct concepts, motif works to emphasize the main themes of the work.