What do we call these huge areas of land?

Continents is correct — that's the word for the seven huge landmasses that make up most of Earth's dry land, each home to many different countries, mountains, rivers, and ecosystems.

Islands are much smaller pieces of land surrounded by water, and rivers are flowing bodies of water, not land at all — neither describes these giant continent-sized areas.

Long ago, all seven continents were joined together as one giant landmass called Pangaea, and they've been slowly drifting apart ever since, moving only a few centimeters each year — about as fast as your fingernails grow.