Kitniyot literally means small things.  They include rice, corn, lentils, beans and peanuts.  At soime point in the Middle Ages, Ashkenazi Jews stopped eating kitniyot during Pesach.  

Conservative and Orthodox Rabbis in Israel have permitted eating kitniyot, as have some Conservative Rabbis in the United States.  Nevertheless, it remains a wide-spread custom within Ashkenazi Jewry to avoid kitniyot during Pesach.  

haggadah Section: Introduction
Source: original