For an excellent article on this subject, see http://www.star-k.com/kashrus/kk-passover-kitniyos.htm.  Parts of this article are used as source material in this section.

Kitniot is popularly translated as legumes – but this is not precise.  The Shulchan Aruch in Orach Chaim 453 defines Kitniot as those seeds and grains that can be cooked and baked in a fashion similar to the five true grains that can become Chametz:  Some examples are rice, corn, peas, mustard seed, and the whole bean family (i.e. kidney, lima, garbanzo, etc.).  The Torah term for the fermentation of barley, rye, oats, wheat, and spelt is chimutz.  The term given for fermentation of Kitniot is sirchan.

The Beit Yosef  permits Kitniot usage on Pesach, while the Rema forbids it.   Hence, Sephardi Jews commonly use Kitniot on Pesach (the normative Seattle Sephardic practice is to refrain from having rice on Pesach), while Ashkenazi Jews follow the Rema’s opinion forbidding them.

Some General Guidelines

• Even among those who do not eat Kitniot on Pesach, the restriction is not all embracing.  Kitniot, unlike Chametz, need not be sold and one can derive benefit from them.  A person can use Kitniot for non-eating purposes, such as fuel for candles, pet food, or medications.  As explained below, it is important to make certain that what appears to be Kitniot is truly only Kitniot and not a Chametz admixture.  In cases of doubt or great need, consult your rabbi. 
• Minhagim (customs) surrounding the use of Kitniot and their derivatives vary greatly among (and sometimes even within) different communities.  
• Sephardim who eat Kitniot have very strict rules about checking them for admixtures.  Please consult your rabbi regarding proper practice.
• While differing opinions exist regarding Kitniot derivatives such as oils, the common practice among Ashkenazim in the United States is to be strict about eating these products.  
• Mainstream major American kashruth agencies do not certify Passover products containing Kitniot derivatives.  However, many Passover products from Israel and Europe (especially France), are designated “kasher l’Pesach l’ochlei Kitniot” (kosher for Passover for those who eat Kitniot) or otherwise contain Kitniot.
• Especially in today’s global economy, many products traditionally assumed to be Kitniot or Kitniot derivatives may, in fact, be derived from Chametz or prepared on the same equipment as Chametz.  In many cases, modern industrial processes may include Chametz or its derivatives in processing.  For example, a common enzyme for converting corn into corn syrup comes from Chametz.  
• Quinoa is neither “grain” (able to become Chametz) nor Kitniot and may be eaten by Ashkenazim and Sephardim.
• Because opinions on Kitniot and their derivatives are diverse and complex, it is critical that you know before you eat.  Again, traditions differ, as do circumstances.  Consult your Rabbi for individual questions and regarding sensitivities towards those whose traditions may vary from your own.

  Some Common Kitniot and Kitniot Derivatives (and possibly Chametz)