A knowledge fragment is the unit of information in a Piggydb database. The box which contains the text you are reading now is a fragment. The content of a fragment is usually text like this fragment (Text Fragment), but you can also input a file of arbitrary type as a single fragment which is called a "File Fragment".
I recommend that you keep individual fragments relatively small. Smaller fragments, which focus on single issues, give you more flexibility when you organize and relate them using the tags.
Piggydb provides you with various ways to view organized fragments. This is one of its advantages when compared to other systems, such as wikis or blogs.