As opposed to a coral cay, a continental island is an island that is built of landmass that is part of a continental shelf.
It is not accumulated coral material like in coral cays, or the coral ring that is left after a volcano sinks down to the ocean like in atolls.
It is simply on the same land as the adjacent continent, a higher part of the continental shelf that is sticking out of the water.
Those islands are often larger than coral cays and atolls. They also tend to be hillier (if it's in a hilly area of course), and they sometimes have more and thicker vegetation.
NOTE: This website is written in British English, which is the English we use in Australia. You will find words like "traveller", "harbour" and "realise", and they are all correct in the language used in Australia.