The main Jewish Holidays include Rosh Hashanah, Yom Kippur, Shemini Atzeret, and Simchat Torah. Other holidays are Shavu'ot, Rosh Hashanah and Sukkot. You can visit http://www.jewfaq.org/holiday0.htm for more details.
I'm not sure of the MOST important ones, or which ones they consider more important than the others, but this link can tell you what the holidays are, when the are, and what they are celebrating/remembering, etc. http://www.hebcal.com/holidays/