A "plot" is the name for the individual 'building zones' in SS2 - those being Residential, Agricultural, Industrial, Commercial, Martial, Recreational, and Municipal. They work kind of similarly to how other "city builder" type videogames do things, it always feels most like the OLD SimCity games to me. You can manually choose which building gets made when you place down a Plot zone, or let the game do it for you. Installing more Addon Packs provides more options for either you or the game itself to draw from, it's MOSTLY just an aesthetic choice.
A "City Plan" is a design for the entire settlement, made up of multiple Plots with the buildings on them the designer selected, and whatever else the original builder put in there (like turrets, walls, water pumps, etc).