Any form of sexual contact where one of the people involved did not consent to the contact is classified as sexual assault. In some cases, such as those involving children, the person being assaulted doesn't know or understand what is happening. In other cases, the person being assaulted may know and understand what is happening, but may be too afraid to object. In either case, because they did not consent to what happened, that makes it sexual assault.
People who commit sexual abuse can be very close to the person they abused, such as family members, caregivers, or coaches. They may have a professional relationship with the person they assault, such as clergy, doctors, teachers, or other professionals. The law takes an especially dim view of people who take advantage of their position of trust to commit sexual assault. Then again, abusers may be complete strangers.
Sexual abuse is a traumatic event that may sometimes continue for years. Survivors can be left traumatized, sometimes to the point of not being able to live a normal life. In some cases, the perpetrator is charged with a criminal offence and goes to jail. In others, the attacker is never brought to justice. And in some cases—even many years after the abuse—survivors sue their attackers in civil (as opposed to criminal) court.