The aspect ratio of an image is the relationship between the width and height.
Generally speaking aspect ratios are express as two numbers with a semi colon between them.
For example a square aspect ratio would be 1:1 whereas classic 35mm film is 3:2.
Here are some common aspect ratios
Early television & large-format computer monitors
Traditional television & computer monitor standard
Academy standard film aspect ratio
√2:1 ~1.4142:1, ISO 216 paper sizes (A4)
IMAX motion picture film format
Classic 35 mm still photographic film
1.6:1 (8:5) (aka 16:10)
A common computer screen ratio.
The golden ratio
A common European widescreen std.; Native Super 16 mm film.
1.77:1 or 1.78:1 (16:9)
HD video std.; U.S. digital broadcast TV std.
A common US widescreen cinema standard
A current widescreen cinema standard