A preferment is a separately fermented portion of a bread formula that is mixed into the final dough. It contributes flavor and fermentation activity. Commercial-yeast versions include poolish and biga; a levain is built from sourdough culture.
Preferments differ in hydration, yeast quantity and ripening time. They are not interchangeable by weight without accounting for the flour, water and leavening they contribute.
Reference: Further reading.