The following chart may help explain how we are related to our cousins.
If someone walked up to you and said “Hi, I’m your third cousin, once removed,” would you know what they meant? Most of us don’t think about our relationships in such exact terms (“cousin” seems good enough), so many of us aren’t very familiar with what these words mean. When working on your family history, however, it’s important to understand the various types of cousin relationships.
First cousins are the people in your family who have two of the same grandparents as you and represent what most people mean when they refer to someone as a cousin.
Second cousins have the same great-grandparents as you, but not the same grandparents.
Third cousins have in common two great-great-grandparents and their ancestors and so on.
When cousins descend from common ancestors by a different number of generations they are called “removed.” Once removed means there is a difference of one generation. Your mother’s first cousin would be your first cousin, once removed. She is one generation younger than your grandparents and you are two generations younger than your grandparents.
Twice removed means that there is a two-generation difference. Your grandmother’s first cousin would be your first cousin, twice removed because you are separated by two generations.
Just to complicate matters, there are also many cases of double cousins. This situation usually occurs when siblings from one family marry siblings from another family. The resulting children, grandchildren, etc. are double cousins, because they share all four ancestors in common. These types of relationships can be difficult to determine and it is usually easiest to chart them one at a time (through one family line and then through the other line).
Source: Kimberly Powell, About.com