Total conversion is a term used to describe mods that replace vanilla Minecraft's mechanics and progression with their own, greatly changing the way the game is played.
The first major total conversion mod was Better Than Wolves (it was not originally created as a total conversion mod, but it later morphed into one). Other popular total conversion mods include TerraFirmaCraft, GregTech 5 and 6, DivineRPG, and The Mists of RioV.
Kitchensink modpacks generally do not include total conversion mods, but there are a few modpacks that include them. These modpacks usually use the total conversion mod as the focus of the modpack and what everything else is balanced off. Examples include TerraFirmaPunk and GT New Horizons.