The Food and Drug Administration (FDA) defines maltodextrins, (C6H10O5)n•H2O, CAS Reg. No. 9050-36-6, as nonsweet, nutritive saccharide polymers that consist of D-glucose units linked primarily by alpha-1-4 bonds, having a DE less than 20. They are prepared as a white powder by partial hydrolysis of corn starch with safe and suitable acids and/or enzymes. Maltodextrins are generally recognized as safe (GRAS) as direct human food ingredients at levels consistent with current good manufacturing practices.
After reading that, I still don't have a clue as to what it is. I've seen it listed in alot of things. From everything as "maltodextrin (vinegar powder)" to "maltodextrin (corn)".