Not a lot of plot details have been revealed, but the Fox Searchlight site has some info from a press release:
"THE TREE OF LIFE is the impressionistic story of a Midwestern family in the 1950's chronicling the journey of the eldest son, Jack (played as an adult by Sean Penn), through the innocence of childhood to his disillusioned adult years - trying to reconcile the complicated relationship with his father (Brad Pitt). Jack finds himself a lost soul in the modern world, seeking answers to the origins and meaning of life while questioning the existence of faith."
There's always a significant gap between Malick's work. It's been five years since his last film, The New World. Two decade passed between Days of Heaven and The Thin Red Line. Malick seems to have been stricken with inspiration recently though, as he's currently shooting another film in Oklahoma, starring Ben Affleck, Javier Bardem, and Rachel McAdams. Look for that one in theaters after your grandchildren graduate college.