Barnard's Loop is a large shell or ring of interstellar gas. It is so large in the sky that if you held out your arm to its full length your hand couldn't cover the full nebula. The shell is expanding at 50 miles/sec (yup... miles/sec, not miles/hr) and its center is located near the Orion Nebula. The shell glows faintly in the optical and is easy to see at radio wavelengths due to the number of ions in the shell. Astronomers believe the shell is a result of the numerous supernovae explosions that have occurred in the region since star formation began some tens of millions of years ago.
M78 is a bright nebula of diffuse interstellar gas. It's a rather small, dim object that requires a telescope or a good set of binoculars to see. The glow is a result of the young stars that have recently formed out of the dark, molecular cloud just behind M78. That is, M78 is the by product of the stars one sees inside the nebula.
The 'M' in M78 stands for Messier who compiled a catalog of over 100 bright nebula or galaxies in the late 18th century. [In comparison, one finds tens of thousands of less famous, less bright objects in today's catalog of nebulae or galaxies.] Messier's objects are some of the most famous objects amateur and professional astronomers observe.