Here's the quick & dirty explanation:
LensesAllows you to simplify your screen layout for your workflow, so it only contains the stuff you're concerned about for a particular task. In other words, you can have a separate lense for tracking/mixing/mastering etc. Any modules you don't use (or don't use for a particular task) you remove, so your screen is uncluttered and you can find the relevant options quicker.
Lenses are global - i.e. once you've set them up, they're available regardless of what project you have open.
ScreensetsThese are project based, so you configure the screen layouts based on what that project needs.
For example, you could have screensets that focus on a particular instrument, or just have the buses in view etc - but it relates only to that project.