I always answer this question by first asking what kind of dogs have you had, because it helps to have a frame of reference. I have had Dachshunds, a Lab-Pointer cross, a German Shepherd - Collie cross, Dobermans, and a German Shorthair Pointer. From my experience, the Briard is most like our German Shepherd-Collie cross. One of our trainers works predominently with German Shepherds and he finds many similarities in the traits and personalities of the two breeds.
The AKC Briard standard says it well: "He is a dog of heart, with spirit and initiative, wise and fearless with no trace of timidity. Intelligent, easily trained, faithful, gentle and obedient, the Briard possesses an excellent memory and an ardent desire to please his master. He retains a high degree of his ancestral instinct to guard home and master. Although he is reserved with strangers, he is loving and loyal to those he knows. Some will display a certain independence."