Hydrogen bond
"A type of weak Interaction between most electronegative elements of periodic table (F,O,N) and hydrogen is called hydrogen bond"
Hydrogen bond is weaker bond than covalent bond but stronger (5 to 30 kJ/mole) than Van der waals force of attaractions. When many hydrogen bonds can form between two molecules (or parts of the same molecule), the resulting union can be sufficiently strong as to be quite stable. the strongest hydrogen bonds are linear - the two electronegativfe atoms  and the hydrogen between them lie in a straight line. 
 Why only F,O,N is formed hydrogen bond?
because of -
a) their small size 
b) highest electronegativity
c) absence of d - orbital
d) repulsion between lone pair and electronegative atom
Thus, "The small sizes of nitrogen, oxygen, and fluorine are essential to H bonding for two reasons. One is that it makes those atoms electronegative that their covalently bonded H is highly positive. Other reason is that it allows the lone pair on the other oxygen, nitrogen, or fluorine to come close to the H. Also, hydrogen bonding has a profound impact in many systems." 

Bilogical Application:

• hold the two strands of the DNA double helix together
• hold polypeptides together in such secondary structures as the alpha helix and the beta conformation;
• help enzymes bind to their substrate [Example];
• help antibodies bind to their antigen
• help transcription factors bind to each other;
• help transcription factors bind to DNA

Types of hydrogen bond:

Inter molecular hydrogen bond:

"Hydrogen bond between two molecules" 

Intra molecular hydrogen bond

"hydrogen bond within a molecule"

Inter molecular hydrogen bond > Intra molecular hydrogen bond