Working in Blender is great, but eventually, you'll want to make the things you create viewable in places other than Blender's 3D View. You may want to have a printable still image of a scene, or a movie of your character falling down a flight of stairs, or you may want to export the geometry and textures of a model for use in a video game. In these situations, you want to export or render.
Exporting takes your 3D data from Blender and restructures it so that other 3D programs can understand it. There are two primary reasons why you'd want to export to a 3D file format other than Blender's
.blend format. The most common is to do additional editing in another program. For example, if you're working as a modeler on a large project, chances are good that whoever hired you, unfortunately, is not using Blender, so you'll probably need to save it in a format that fits into their pipeline and is understood by their tools.
The other reason for exporting is for video games. Many games have a public specification of the format they use for loading 3D data. Blender can export in many of these formats, allowing you to create custom characters and sets.
With only a few exceptions, all of Blender's exporters are scripts ...