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!