Developed in 1928 by Otto Diels and Kurt Alder, the Diels-Alder Reaction is an extremely important reaction for synthetic organic chemistry. It is an addition reaction where a conjugated diene reacts with a dienophile in a pericyclic reaction involving the simultaneous movement of 3 bonding electron pairs. The Diels-Alder reaction find common use among synthetic chemists because it is a regioselective and stereoselective means of creating a cyclic or bicyclic molecule. Diels and Alder were awarded the Nobel Prize for Chemistry in 1950.
A Typical Diels-Alder Reaction:All carbons explicitly marked for clarity
1,3-Butadiene Cyclohexene
C Ethene C
// C / \
C || C C
| + || => || |
C C C C
\\ \ /
C C
Thanks to vuo for constructive critcism/corrections, I'll try to expand on this later once school lets up enough.
In the meantime, Kurt Alder's Nobel Lecture describing the reaction in more detail can be found here: http://nobelprize.org/nobel_prizes/chemistry/laureates/1950/alder-lecture.pdf