M17 is a typical member of the class of emission nebulas. It strongly glows in the light of ionized hydrogen and oxygen. It's a craddle of newly born stars, the brightest of them - although hidden in the interior of the gas clouds - are the source of the radiation which causes the nebula's glow.
A bigger version can be seen at http://www.pbase.com/gbachmayer/image/94885740/original