Conjugation of Double Bonds
A diene is said to be conjugated when its double bonds are not directly next to each other, but rather separated by a single bond in between them (CH2=CH-CH=CH2).
Conjugated dienes are particularly stable due to the delocalization of the pi electrons along sp2 hybridized orbitals, and they also tend to undergo reactions atypical of double bond chemistry. For instance, chlorine can add to 1,3-butadiene (CH2=CH-CH=CH2) to yield a mixture of 3,4-dichloro-1-butene (ClCH2-CHCl-CH=CH2) and 1,4-dichloro-2-butene (ClCH2-CH=CH-CH2Cl). These are known as 1,2 addition and 1,4 addition, respectively. 1,2-addition is favored in mild reaction (irreversible) conditions (the kinetically preferred product) and 1,4-addition is favored in harsher reaction (reversible) conditions (which results in the thermodynamically preferred product).
Another interesting conjugated diene reaction is the Diels-Alder reaction, in which a conjugated diene reacts with an alkene (called a "dienophile" in this case) in order to yield a cyclic product. In Diels-Alder type reactions, the substituent on the dienophile will go into either an exo or endo position in the final product. ("Endo" in this case meaning trans to the substituents on the ends of the diene, and "exo" meaning the same thing, only cis.)