What is Superposition?

Introduction to Quantum Computing

When vectors are expanded in the standard basis and it contains more than one non-zero component, that linear combination of the basis vectors are called a superposition.

This is written like α|0> +β|1> and this two-dimensional, complex vector is called a superposition state. 

A special superposition state called the plus state is |+> = (1/√2)|0> + (1/√2)|1> as this is addition.

A special superposition state called the minusstate is |-> = (1/√2)|0> - (1/√2)|1> as this is subtraction.