One-Piece Toilets are molded with the tank and bowl connected into a single seamless piece. These toilets are generally heavier and the seamless design allows for easy cleaning. These toilets are moderately difficult to install as they can be a bit heavier and bulkier to move being one single piece.
Two-Piece Toilets are the most commonly used toilets in the US. They are sculpted with the tank and bowl as two different parts which are assembled together during installation. These toilets are the easiest to install as it is easier to maneuver two separate pieces if you are installing a toilet alone.