Well it seems those two terms are the most popular ones since 11.04. What truly are the differences and similarities between them (lenses and scopes)?
You use a lens to look at the data returned by a scope.
For instance, you have one single music lens, which uses different scopes to display its results: local music, music in the Ubuntu One Music Store, music from the Amazon MP3 Store, etc.
The main idea is that there can be lots of different, but very similar, data providers. The scope is used to make them act the same way and the lens is used to display that data with categories, filters, etc.