When we refer to light, we usually mean a very small part of the whole electromagnetic spectrum which we can see with the human eye. Sometimes this type of light is called visible, or optical, light.
Light is complicated as it can behave as both a wave and as a particle.
The particle form of light is known as a photon and these can be thought of as packets of light, carrying energy. It is counting these photons hitting cameras (containing CCDs) which we attach to telescopes which allow us to see how bright an object is - or as it is known in astronomy, the object's flux.
When light behaves as a wave, it still carries energy, but it has a whole bunch of other properties to help us define it.
Every wave (whether it be light, water, sound or anything else) has the following features:
When we talk about visible light the range of wavelengths is very small - only about 400-700 nm (nanometers - 1 nanometer is a billionth of a meter - a human hair is about 100,000 nanometers!). The wavelength of visible light also decides what colour it is with the longest wavelengths being red (~700nm), and the shortest being blue (~400nm). In between these is every colour of the rainbow - to find out more about the colours of visible light follow this link.
All light travels at the same speed, known as the speed of light, c and in space, which is a vacuum (meaning there is no air or gas, or in fact anything!) this speed is 300,000 km/s! Light is the fastest thing in the Universe - nothing can travel faster, and in astronomy we sometimes use it's speed to measure distances. We define is as the distance light could travel in one year, and we call that distance a light year.