A D Minor chord contains the notes D, A, F. It is built from the minor formula 1 - ♭3 - 5, with D as the root.
What is the difference between D major and D minor?
Only one note. D major contains F♯; D minor flattens that to F natural. On the guitar it's a single fingertip moving down one fret on the high E string — which is what makes switching between the two such a useful early exercise.