A asset is generic name for any type of content that is needed in the game. Examples are audio, video, models, materials, textures, animations and localization data to name a few. Native code is normally not handled as assets but script files are. The asset pipeline is the part that takes the data from the tools used to create it and then convert it into the final data format used by the game engine.
These are created by the content creators of the game. Often created in specialized tools such as Maya or Photoshop. This data is often stored in the tool-specific format as the application specific data is needed to make it easy to modify the data. Ex are all the controllers in Maya for animations or all the layers in Photoshop. To create a intermediate assets from a source asset a exporter is used that create the intermediate asset.
This type of asset is created by using a exporter from the tool that is used to create the source assets.
The final asset is one that is built for a specific target platforms. Each platform has it's own optimal format for different types and usage of assets. One platform might have custom hardware to decompress certain texture formats faster for example. The final asset is also optimized to load as fast as possible on the target platform.