like sigweed said, it's a nebula, a MASSIVE cloud of dust that will form into stars. the parts where it's thicker are where the dust particles are being pulled together by gravity. "fingers" form from the columns as the dust comes together and the stars will be born eventually.
and when i say massive, i mean massive. many lightyears across. there are some cool pictures here:
http://hubblesite.org/
too bad politics are forcing NASA to let Hubble die. there won't be a replacement telescope until 2010 or so.