A grand-aunt or grand-uncle is the sibling of your grandparent, while great-aunts and great-uncles are further removed. However, "grand" and "great" are often used interchangeably.
Just as a person's parents' parents are grandparents, the parents' aunts and uncles are grand-aunts and uncles. Likewise, a person's great-grandparents' siblings are great-grandaunts, and additional greats are added as the generational gap increases. Although grand-aunt or uncle is the official title, grand-aunts and uncles are often referred to as great-aunts and uncles, and many families simply use the title "aunt" for grand- and great-grandaunts and uncles. The title of great or grand-aunt also applies to sister-in-laws, not just sisters by blood.