The Lesson
An expression is a term or several terms that are added together or subtracted.Dictionary Definition
The Oxford English Dictionary defines an expression as "a collection of symbols together expressing an algebraical quantity."Real Examples of Expressions in Algebra
It is easier to understand expressions with examples.
An expression can be a single term:

An expression can several terms that are added together or subtracted:
Each term is separated by an addition (+) or subtaction () operator.
From Expressions to Equations
An equation is made when one expression is made equal to another expression.An algebraic equation has one expression either side of an = sign.