What exactly is a "lens"?

• https://wiki.ubuntu.com/Unity/Lenses
• What lenses for Unity are available?
• http://www.omgubuntu.co.uk/2011/04/five-neat-unity-lenses-in-development/

It is a graphical interface for a task, it is also linked in with the dash for search results. for example when clicking the application lens it shows the apps in a nice, slick interface , with other results for like app to download and most frequent apps.

If you hover over the "lenses" tag below your question, you'll see this succinct description:

Lenses are elements of the Unity Dash that provide a UI to search both the web and application data.