Quote from "The Universal Book of Astronomy" by David Darling, Wiley Publications
Quote:
A small bright nebula in a star forming region, created when fast moving jets of material from a newborn star collide with the interstellar medium. As the bipolar flow from a young star plows into the surrounding gas, it generates strong shock waves that heat and ionize the gas. In the cooling gas behind the shock front, electrons and ions recombine to give an emission line spectrum chacteristic of Herbig-Haro objects. They are named after the American astronomer George Hergig (1920 - ) and Guillermo Haro who discovered the first three such objects in 1946 - 1947 in images of the nebula NGC1999 in Orion. All known Herbig-Haro objects have been found within the boundaries of dark clouds and are strong sources of infrared."
End Quote
Hope that helps
This attachment, a scan from the same book might illustrate it better