Humility is the quality of being humble. If you meet one of your heroes and feel nervous and in awe for her, you are experiencing humility.
Humility comes from the Latin word humilis, which literally means low. If you feel humility in front of someone, you feel small in the scheme of things — that you are just a simple, insignificant person. Someone who spends his life taking care of others shows humility. Although it's related to humiliation (which makes you feel low in a bad way), humility is usually used to talk about a lack of ego.