Scientists have just seen for the first time one of the most important physical interactions in our world — the special type of bond called the hydrogen bond that holds our DNA together and gives water its unique properties, including surface tension.
When a tiny hydrogen atom is in a molecule with a much bigger atom, like nitrogen or oxygen (for example, in water), that larger atom, pulls away some of the negative charge from the smaller one, giving the smaller one a slightly positive charge on one edge. That slightly positive charge is electrically attracted to the slightly negative charge on the large atom of another molecule.
Read more: http://www.businessinsider.com/first-images-of-a-hydrogen-bond-2013-9#ixzz2ggLxsu2i
Also has some images of covalent bonds.