Energetic radiation from Sigma Orionis, the naked-eye star at the top of the picture, illuminates the surface of an otherwise invisible dusty cloud, exciting the distinctive red emission from hydrogen. From this dark cloud projects yet more dust, which has the shape of the head of the horse, seen in silhouette against the glowing background.
Here is a wide angle view of the Horsehead Nebula and NGC 2024 in Orion.
Images by David Malin, AAO.