Competitive Pokemon. What are the "Tiers" and "Formats?"?

Question

Self explanatory. Please go in-depth. I'm a bit of a newcomer, and this will help me out greatly. Thank you.

Comments

If you'd like to acquaint yourself with these terms more directly, spend a bit of time on Showdown: https://pokemonshowdown.com/
Most of this terminology derives from Showdown, as it is the centrepiece of nearly everything related to competitive Pokemon. In doing this, you'll also come across Smogon, which is the central body for fan-made tiers and formats, and their governance.

Answer 1

All Tiers are Formats, but not all Formats are Tiers. :P

Formats in the sense of competitive Pokémon are battling with a specific rule-set that both players agree with. An example would be Monotype, in which you can only use Pokémon of the same Type. Here is a list of Smogon's formats.

Tiers are formats that group Pokémon together based off of usage stats and banlists. The higher Tiers generally have Pokémon that perform better overall (or better in the current meta) than the others below them. Here is a list of Smogon's Tiers.

Hope I Helped!