Extensions

Extension allows you to adds new features to the software. These are small programs that make things easier for modeling, animation, or rendering. You can turn them on or off by going to Edit > Preferences > Add-ons in Blender. Many extensions are free and can make your work faster and easier!

Add - ons

Addons allows you to get more out of blender.
Its like downloading and customizing the software
according to your needs, rather than getting all the feature
out of which you hardly use like 60% of the features