What is Types of rotors?

A molecule with three equal principal moments of inertia is a spherical top ($\ce{CH4}$, $\ce{SF6}$); with two equal, a symmetric top ($\ce{NH3}$, $\ce{CHCl3}$, $\ce{C6H6}$); with all three different, an asymmetric top ($\ce{H2O}$, most molecules); linear molecules have one moment zero.