From Citizendium, the Citizens' Compendium
Borrowing from Friedrich Nietzsche's process of genealogy, deconstruction seeks to read a text against itself. This is done through the revealing of the signs every sign is pointing to, in infinite reference. Deconstruction stipulates that a subject of interest (such as a text) can be better understood through this manner. Through a word Jacques Derrida is hesitant to use, deconstruction allows the reconstruction of new meanings or better understandings.
Essentially, Deconstruction tries to show that subjects of interest—texts, traditions, societies, beliefs, etc—do not have definable meanings. There is the understanding that all signs and symbols are required to be more than they appear to be in order to have meaning. That signs and symbols depend on something Derrida calls différance—a neologism created by Derrida that puns on the French word différence, which has the same meaning in French as in English.
Différance relies on the space, the difference, between every sign. This space exists in the referral between signs. For example, a “dog” is different than a “canine” even though they refer to the same type of animal. The difference between “dog” and “canine” gives each its unique qualities. Interestingly, these unique qualities exist only because there is not a unique sign and symbol for any one subject of interest. One way to describe Différance: The difference of difference is Différance. To put it more simply (a hard task with Derrida), it is only by the distinction of symbols from one another that gives rise to their individual capacity. However, these symbols can not stand on their own -- they require other symbols and signs to give them their Différance.