Distinguish between Co and CO ?

CO is a molecule made up of a carbon atom, C, and an oxygen atom, O. The common name for CO is Carbon monoxide and it is poisonous.

Co is an elemental symbol for the element Cobalt. Cobalt is #27 on the periodic table. It is a metal.

So to sum it up,

CO is a molecule made up 2 atoms, Co is an element.