Export is a command usually found within a programs File menu (File → Export...). It is similar to the File → Save As... command, but is typically used for more specific purposes. For example, instead of simply saving a file with a different name or different format, Export might be used to save parts of a file, create a backup copy of a file, or save a file with customized settings.
Since the Export command is only used for specific purposes, it is not available in all applications. For example, most text editors do not include an Export feature because text documents do not contain anything specific to export. Instead, the Export command is often found in multimedia programs, like photo and video editing applications. For example, Adobe Photoshop includes an Export command that allows users to export vector paths within an image as Adobe Illustrator files. Apple QuickTime Player allows users to export videos to multiple formats and provides advanced options for choosing the compression type, frame rate, video dimensions, and other settings. QuickTimes Save As... command only allows users to save movies in the standard QuickTime (.MOV) format.
A programs Export command can be a useful tool if you need to save elements within a file or if you would like to save a version of the file with customized settings. Therefore, if a programs Save As... command doesnt have the options you are looking for, select File → Export... and you might find exactly what you need.
Many programs also include an Import command, which is used for opening specific types of files.