You'll find dozens of articles and books on the subject.
But one sentence embodies its essence: Reconstructionists see Judaism as an evolving religious civilization.
Evolving -- because Reconstructionists engage in an ongoing dialog designed to keep Jewish principles and practices meaningful to the world in which we live, rather than set in stone.
Religious -- because we embrace the core of Judaism based upon our shared religious inheritance.
Civilization -- because in addition to these religious underpinnings, Judaism includes our far broader cultural, historical and intellectual heritage.
Reconstructionism doesn't easily fit into a "straight line" model of Judaism from "most traditionally observant" (Orthodox, Chasidic) to "least traditionally observant" (Reform). Instead, we say that tradition has "a vote, but not a veto" as we examine how best to live as Jews in the twenty-first century.
One example: the first girl to have a Bat Mitzvah was the daughter of the founder of Reconstructionism.