Chastity and Abstinence are often used interchangeably these days.

Actually... 

Chastity  is a spiritual power, something deep within yourself.

It is a lifestyle, a philosophy of life.

Abstinence  is a behavior used to achieve Chastity.

Abstinence means the avoidance of certain substances and activities which can be detrimental, dangerous, or deadly to a person's well-being.