Wisegeek defines it this way- Cafe au lait is a French term which means “coffee on milk.” It is used to refer to a beverage made with strong, fresh coffee and hot milk. Many former French colonies offer this drink as a staple accompaniment to breakfast, as do regions with large French populations. The beverage should not be confused with a latte, a drink made with espresso, not strong coffee. This can lead to confusion in a coffeehouse especially, so do not be afraid to be specific about what you are asking for.
To make cafe au lait, milk is scalded and poured into a cup simultaneously with coffee. Because the two are poured together, rather than layered, the resulting drink is thoroughly mixed. It can be drunk plain or sweetened, depending on personal taste. In some regions, chicory is mixed with the coffee beans for a characteristic bitter bite. This is especially common in Louisiana.
Many nations have a variation of the cafe au lait, which is in an intermediate zone between brewed black coffee and blended coffee drinks. All of the variations mix fresh coffee with hot milk, and the name usually indicates this. In Germany, for example, it is known as Milchkaffee, or “milk coffee.” Some coffee shops refer to the beverage as a misto, or blend.
Happy National Cafe' Au Lait Day!