While the aim of Wikipedia is to be a online encyclopedia, Diffen's scope is to be an encyclopedia of comparisons.
It's a somewhat narrower scope because the articles on Diffen are not on a specific topic but on the difference between or comparison of any 2 things. For example, while Wikipedia has articles on LCD screens and Plasma screens, Diffen has an article on the difference between LCD and plasma.
While Wikipedia articles do often mention differences between similar things, and there are also some comparison articles — such as comparison of text editors — Diffen articles focus on comparing any two "entities.
Relationship between Diffen and Wikipedia
Diffen is not a part of the WikiMedia Foundation. There is no formal relationship between Diffen and Wikipedia.
As an information resource, Diffen complements Wikipedia for researchers interested in differences and comparisons.