I was intrigued by this question and did some research for myself, too. A review of several geography books, including Geography for Dummies, not a very flattering title, and Don't Know Much About Geography, by Kenneth C. Davies, as well as several websites, left me quite dissatisfied.
What I have learned is that a continent is a large land mass surrounded by water. Any land mass not large enough to be considered a continent is an island. Hmmm.... What does that mean? There are no scientific criteria whatsoever. We cannot say that anything over X number of square miles is a continent and anything under X square miles is an island. There are no geographical features involved except for the requirement of a land mass, which is required for both.
I did learn that in the ancient world, there were three landmasses considered to be continents, which were Europe, Asia, and Africa (Davies). But even though we have discovered the other four, I am not sure our geographic concept of this distinction is any less primitive than that of the ancient world!
A Continent is one of the seven great massed of dry land on Earth. These seven continents are North America, south America, Europe, Asia, Australia and Antarctica.
An island is a body of dry land , smaller than a continent, surrounded by water on all sides.
In the past continents were thought to be surrounded by water from all sides, and to that extent were supposed to be very big island. But now we know that only Australia and Antarctica are completely surrounded by sea on all sides. Africa, after building of Suez Canal can be said to be surrounded by sea on all sides. North America and south America taken together are surrounded by sea from all sides but taken independently they cannot be said to be completely surrounded by sea. If we consider Panama Canal to be a body of water across which ships can sail, then these two continents may also be considered to be completely surrounded by sea. Asia and Europe in reality a large common mass of land with a very long common land border. These two are considered to be separate continents because of historical reasons. Some scholars have suggested that Europe and Asia should be treated as a single continent with the name Eurasia.
There are actually only 7 continents on earth while there are many more islands . The 7 continents are North America , South America , Africa , Asia , Europe , Australia , and Antarctica . Also , islands are small body of lands that its borders only water . Continents though , are much bigger for it consists multiple countries .