Definition of dray. : a vehicle used to haul goods; especially : a strong cart or wagon without sides.
The main difference between an equation and a function is the fact that an equation usually deals with a single input, whereas, a function can have numerous inputs. In mathematics, an equation is used to denote the equality between two expressions.
An expression is basically an incomplete mathematical sentence. It is like any normal phrase in the English language. Compared to expressions, equations are more complete. They are homologous to what completely structured English sentences are.