How does the BLS classify people who are "not in the labor force," and what people are often in this category? 

If an individual surveyed (that is, who is age 16 or over and not institutionalized) is neither employed or unemployed, that individual is measured "not in the labor force" according to the  BLS.  Often people in this category are in school, retired, disabled, or taking care of people in  their households and communaities.