There are actually 19 Hawaiian Islands. There are 8 main islands and 4 that people usually think of. The more commonly known islands are:
Oahu: This is where Honolulu is, making it the most accessible island because it has the biggest city and airport of all the Hawaiian islands. There are a lot of famous tourist destinations here, including Pearl Harbor and Waikiki beach.
Maui: The second biggest island in terms of land, and the third largest in terms of population.
Hawaii - The Big Island: The largest (by far) of the islands, and the second largest population. This is also the newest island and it is still growing. The Big Island is the home of Volcanoes National Park, which is one of the most unique places in the world, where you can walk right up to flowing hot lava. The Big Island, due to its size and smaller population, is the most affordable island in many ways, although it also requires the most driving.
Kauai: The fourth largest island in terms of both land size and population (with about 58,000 people). The other islands are significantly smaller, with the next highest population being fewer than 8,000 people.