Without going too much into detail, Fatbikes are generally those that can support a 4" or wider tire, typically used for flotation over soft, loose, mucky or sandy surfaces. To put that in comparison, take your average skinny-tire bike of old and double the tire width. Wider tires float over stuff that would make skinner tires dig in and stop. Plus, they're loads of fun in a raucous, monster-trucky kind of way. Plus bikes typically have tires between 2.5" and 3.5", and have been seen as good compromise between the float-but-slow of fat tires and the fast-but-easily-stuck of skinny tires. Lots of bikepacking rigs sport them now, as well as many trail bikes. Q-factor is the width between the cranks. As tire size increases, the cranks need to get wider to fit around the tires. Many people find the wider stance to be a bit bowlegged-y. I personally don't mind, but some do.
If you just want a to-the-pool commuter for pavement riding, most anything will do, Walmart bikes included. If you want to get into cycling at even a modest level, I would stay far away from department store bikes, as they are typically anvil-heavy, poorly constructed and break quickly. Shop used bikes or go to your local shop and source some deals, you'll be glad you did.