What is the differance between Linked and Appended data from external .blend files?
The difference is simple.
Link: Link copies the object from the blend file, much like Alt-D does for meshes. When it is pulled in, the mesh/material whatever cannot be edited in the file. Changes in the original file will cross apply to it every where. This is useful for production assets which need to be the same in ever shot.
The object will be copied into the new file, there will be no link, much like importing an
Appended data is just copied into a blend file.
Linked data will continue to update if the original data is updated. For example, if you link a red material from blend 1 into blend 2 the material will appear red. If you then update the original material in blend 1 to be blue (and then save blend 1) the material will also appear to be blue in blend 2 (once blend 2 is reloaded).
Linking allows organisation when creating complex scenes. Instead of modelling many props, rigging characters, and then animating in a single file, which might become complex, you can split your project into separate files and then link them together to render.