The seven continents form the total landmass on earth. In fact, Earth is about 71% water and only 29% land, though billions of years ago, there was only a single massive landmass called Pangaea in which all the seven continents of the world were combined. It was only due to plate tectonics that they eventually broke apart and separated to form the seven continents. Some of these continents are connected to each other, while some aren't.
Moreover, each continent has a different number of countries forming them along with the differing sizes. These continents include: Asia, Africa, Europe, Australia, North America, South America, and Antarctica. Interestingly, all the continents start and end with the same alphabet, that is, only if you consider North and South America to be a single continent. Further, research even says that North America and Europe still move apart at about 7cm rate every year.
When geographers identify a continent, they include all the islands associated with it, for example, Japan is a part of the continent of Asia, while Greenland and other islands in Caribbean Sea are considered to be a part of the continent of North America. Another thing that geographers consider when it comes down to continents is that they believe continents are culturally distinct. For example, Europe and Asia are part of a single piece of land, known as Eurasia, but they are linguistically and ethnically different.