Aperture Size

In the world of telescopes, big is best. Obviously magnifying power is important but another important factor is aperture size. The aperture is the end of the telescope that light actually enters.

The larger the aperture then the more light enters the telescope. The amount of light is proportional to the area of the aperture so, as area = π r² then a telescope with an aperture twice the diameter will collect four times as much light.

This is important when observing very faint objects in the sky, e.g. distant stars.

Another advantage of a large aperture is its effect on resolving power. This is the ability to tell faint objects apart.   

original pic - NASA

The only difference between how these pictures of the Andromeda galaxy were taken is the aperture size of the telescope. Notice how the larger aperture at the end produces brighter, sharper pictures, i.e.. better RESOLUTION

Resolution = the ability to distinguish between two point sources of light. With poor resolution two points blur into one, with good resolution they are easy to tell apart. Bigger aperture = better resolution.