The difference between a theme and a motif can be difficult to distinguish because they are so closely related; but a theme is a dominant idea, issue, or topic of a story or poem. Some popular themes throughout literature are death, love, hate, revenge and friendship. A motif, though, is when a theme repeats itself throughout a story or poem. One might discuss a motif after reading a story and discovering that the same theme is found again and again at different points in the story. It's as if the author wants to highlight the theme's importance as it is demonstrated in different ways. One example of this is found in Gabriel Garcia Marquez's book "One Hundred Years of Solitude". In this book, the theme of solitude is portrayed from one generation to the next from grandparents to grandchildren. Each generation experiences solitude in a different way, but the motif is clearly outlined throughout the book.
In works of literature, a theme is generally considered an overarching truth--a message that serves as the core idea of the story. Themes provide insight (instead of serving as a mere subject), and they usually highlight some perception or point about the nature of humanity, society, the world, etc. Themes may be recurring, universal, stated, or implied.
Motifs, on the other hand, are repeated ideas or objects that frequent a work of literature. They occur as images, action, figures, or sounds and usually carry symbolic significance; that being said, they are not in themselves symbols.
Despite these two terms being very different, they do retain an important connection: a motif can be used by a writer to reinforce or contribute to the theme of a story.
The two terms are related but have totally different meanings. A theme is a dominant idea that the writer wants to convey. Themes tend to be universal ideas such as perception vs. reality, the materialism of society and so on. Where as motifs are reoccurring words and images that suggest particular meanings.
Basically themes are full-fledged ideas that run through an entire text and motifs are are half-formed ideas that are suggested.