Hi Nick,
Above the solar surface (we call this the photosphere) we split the different heights of the atmosphere into different layers depending on their different features. It's a similar idea to the different parts of the Earth's atmosphere (troposphere, tropopause, stratosphere, ozone layer, mesosphere and ionosphere). In the Sun's atmosphere we have, going up in height, the photosphere, chromosphere, transition region and corona.The temperature changes in the different regions which means that each region radiates different parts of the electromagnetic spectrum i.e. they give off different kinds of light, from ultraviolet and infared light to x-rays and radio waves. By using filters on our telescopes we can observe the different the kinds of light and therefore see the different layers of the Sun's atmosphere. For example the photosphere can be seen in visible light, the chromosphere and transition region can be seen in ultraviolet and extreme ultraviolet and the corona can be seen in extreme ultraviolet and X-rays.
Hazel