Showdown Tiers in a nutshell

Showdown uses "usage" based tiers.

NU = Never Used, UU = Under Used, OU = Over Used. Pokemon that are used most frequently are sent to higher tiers.
You can use lower tier mons in higher tiers but not higher tier mons in lower tiers.

Uber is the banlist from all the tiers, but for some stupid reason people decided Ubers needed to be fun too, so they made a banlist for Ubers called Anything Goes

This system is favorable as you can choose between allowing "anything fun" (OU) or you can play lower tiers to give less powerful mons some love (RU)

However, it breaks down when mons get banned to higher tiers or Ubers because of metagame shifts, causing key components to your team to get banned to no fault of your own.

Those last two points are the main difference between Showdown's tiers and Battle Spot's Rating system. Rating Battles allow anything but Mythical and Legendary Pokemon, with Sublegendary allowed (see: https://www.serebii.net/pokemon/legendary.shtml). This means you  never have to worry about parts of your team getting banned, but on the other hand, some of your favorites probably aren't going to be viable.