The Lollipop upgrade was a fairly major update, but depending on what phone you have, you may not have a ton of noticeable or "user facing" changes.
There were a lot of changes "under the hood" so to speak to improve battery life and general OS speed, as well as make things more secure and such, as well as various other improvements that you might not see. There was also a fairly major change in the look and feel of the interface, from the "Holo" design language of 4.0-4.4 versions of Android to the new "Material" design language. Again, depending on your phone, you may not see this as much because every manufacturer has their own skin on Android that puts some of their own spin on the look and feel.
If you want to know the exact changes made, a Google search with your device name and "Lollipop upgrade" will probably find you a few articles that discuss what was changed and updated on your specific device.
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