What we generally call continents are giant landmasses on our planet, nested between the surrounding oceans. There is no exact definition or criterion of what is to be considered a single continent, the seven continents are rather based on traditions and conventions.
The largest of the seven continents is Asia, which in itself is only a part of the Eurasian landmass. Due to historical and cultural reasons, however, Asia is taken separately from Europe when we usually talk about continents. In fact only Antarctica and Australia are disconnected completely from the other landmasses by bodies of water. Asia is separated from Africa by the Isthmus of Suez, while North America connected to South America by the Isthmus of Panama.
These distinctions are only true when we accept the seven continents model. Other approaches count only six or five continents by joining North and South America as America, and also connecting Europe to Asia on the joined landmass of Eurasia.
Asia, being the largest of all, is also the home of the highest mountains in the world, and is also by far the most populous continents of all. The second largest and second most populous landmass is Africa, which is often called the black continent. Australia is the smallest of the seven, and is a single country continent.
Alternative ways to distinguish continents
6 continents model
North America, South America, Africa, Eurasia, Australia, Antarctic
Six continents model
America, Africa, Europe, Asia, Australia, Antarctic
5 continents model
America, Africa, Eurasia, Australia, Antarctic