Just below the left star in Orion's belt is probably the
best known and most easily recognized of the "dark nebula":
The Horsehead. The entire top half of this image is lit up
by the glow of hydrogen under the radiation of powerful
stars. The vertical lines within this glowing red curtain
are caused by powerful magnetic fields funneling the streams
of gas.Between us and this glowing cloud is a great blob of
dust which blocks the light of the cloud behind it, and the
shape looks remarkably like the typical knight in a chess set.
Below and to the left of the horsehead is an area where a bright
star is carving out a hole in the cloud. The light from that star
is reflected by the remaining dust and gas. This blue reflection
nebula is NGC 2023. And so this small section of the sky demonstrates
the three main kinds of nebula: emission (the red section), reflection
(the blue), and dark (the Horsehead itself).

Image data:
Camera: STL-11000
Exposure: Hydrogen-Alpha: 8 x 30 minutes (blended with Luminance & Red)
Luminance: 21 x 10 minutes
RGB: 7 x 10 minutes each
(total = 11 hours)
Telescope: Hyperion 12.5"