In photography industries, cropping means to remove unwanted areas from an image. This process is one of the most basic photo manipulation processes. This process is performed in order to remove an unwanted subject or irrelevant detail from a photo, change its aspect ratio, or to improve the overall composition.
In telephoto photography an image is cropped to magnify the primary subject and further reduce the angle of view when a lens of sufficient focal length to achieve the desired magnification directly is not available.
Cropping is widely considered one of the few editing actions permissible in modern photojournalism along with tonal balance, colour correction and sharpening.
A crop made from the top and bottom of a photograph may produce an aspect which mimics the panoramic format (in photography) and the widescreen format in cinematography and broadcasting. Both of these formats are not cropped as such, rather the product of highly specialized optical configuration and camera design.
Here is a video about cropping:
Video by Lynda.com
Here is a video about cropping in post to improve your images:
Video by sergeremelli