A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, like nitrogen, oxygen or fluorine (thus the name "hydrogen bond", which must not be confused with a covalent bond to hydrogen). The hydrogen must be covalently bonded to another electronegative atom to create the bond. These bonds can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly). This type of bond occurs in both inorganic molecules such as water and organic molecules such as DNA.
Intermolecular hydrogen bonding is responsible for the high boiling point of water (100 °C). This is because of the strong hydrogen bond.
For more, visit the Wikipedia entry for aqueous solution, URL: http://en.wikipedia.org/wiki/Hydrogen_bond