The simple definition is having sex with anyone under the age of consent, with the government under the assumption that anyone under that age can not make intelligent decisions for themselves.

If both lovers are under the age of consent, statutory rape is defined by an age difference, in my state its 4 years.

This way an 18 year old having sex with a 17 year old isn't considered statutory rape. The numerical age difference holds precedence over the age of consent when applicable.