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).