In aviation - Aspect Ratio refers to the shape of the wings of an aircraft, i.e. long and skinny or short and fat.
The simplest expression of AR is span/chord, where span is the distance from wingtip to wingtip and the chord is the distance from the leading edge to the trailing edge of the wing. This works for rectangular wings such as those found on Cessnas and other small airplanes. For tapered and/or swept wings the average chord is needed. In these cases a more useful equation for AR is span^2/area.

Aspect Ratio relates to the performance of a wing, that is to say, its efficiency (at a given speed or speed range). Aerodynamic drag is the chief foe of wing efficiency, and comes in two forms: parasite drag and induced drag. The former is the result of airflow disruption by struts, wheels, fuselage, etc. Induced drag is a consequence of the production of lift. A certain amount of energy goes into moving air in ways that don't lift the aircraft; an example is the tip vortex, where air swirls from the bottom to the top of the wing at the tip. The effects of drag increase as the square of airspeed.
Induced drag varies inversely with aspect ratio, so a wing with infinite span would have zero induced drag. High performance sailplanes, with their very long and skinny wings, have ARs of 30 or higher. The Space Shuttle has an AR of maybe 1.5. Hang gliders have ARs in the 5.5 to 8.5 range.
Low aspect ratios generally work better for wings designed to produce a lot of lift at low speeds, high aspect ratios are better for higher speed applications where minimal lift (beyond supporting the aircraft) is needed. Lower AR wings are generally much more maneuverable than high AR wings as well.
Structural factors limit the attainable aspect ratio using a given material or materials, and the neccesary light weight means that very high AR wings tend to flex or curve dynamically under load, which further complicates designing for efficiency and realizing the theoretical benefits of high AR.