How Am I Related to You?

On my mother's side of the family, we know several generations of relatives.  I can't say I really know how I am related to all my cousins, aunts and uncles.  As I grew older, I began to realize that some of my cousins I call Auntie and Uncle are actually not my Auntie and Uncle.  I was just told to call them that because they were adults when I was a child.  I have one Auntie/Cousin who I think actually understands how we are all related.  She says things like, "he is your cousin once removed" (which I don't really know what that means).  I remember at one of my cousin's weddings, when I was a child, I met some distant cousins, around my age, and their father was Mexican-American.  I was so intrigued by that.  I ran to my mother and yelled out, "I have Mexican cousins!".  She later told me when I was an adult, she was embarrassed, but she said she didn't scold me for yelling since she said I was so thrilled at the discovery.