* Squirrels are rodents (order Rodentia). They are part of the Sciuridae family, which includes ground squirrels, tree squirrels, chipmunks, and marmots.
* Sugar gliders are marsupials (order Diprotodontia). They are part of the Petauridae family, which includes other gliding possums.
While they both have the ability to glide, this is a convergent evolution – meaning they evolved this trait independently due to similar environmental pressures. Their evolutionary lineages diverged long ago, making them more distantly related than, for example, cats and dogs.