Necking includes caressing and embracing. Heads rest on shoulders of the lovers and faces are pressed close to each other. You may see a girl leaning her head on a fellow's shoulder while they sit and hold hands. They may or may not be kissing, but they are necking. If they are not holding hands, an arm may be about the other's waist.
Take a walk through the park on a summer night, or watch the automotblies go by, and on a park bench, or seated in a car, you will see lovers necking. They are respectfully familiar with each other. Most of these young people are intimate friends or lovers. The type of person who would neck with a stranger would go on to petting. It is, as a rule, lovers who care for each other who indulge in necking. To accuse them of petting would be an injustice.
The more you know.