What is attachment and what are the three main attachment styles according to Bowlby and Ainsworth?

Attachment is a homeostatically driven bond between the infant and their primary caregiver, evolutionarily aimed to protect the infant from danger and increase chances of good care. Attachment is thought to be vital for successful development of a coherent self with interpersonal understanding and emotional control and several studies (Spitz, O'Connor) have shown evidence of how severe early disturbances in attachment formation can damage healthy development of the child. It is thought that there are more and less adaptive types of attachment, but every type is thought to have their own strengths and weaknesses.
The three main attachment styles were first discovered and researched by John Bowlby and Mary Ainsworth and consist of: 1. secure attachment (the child shows low separation anxiety when caregiver leaves, explores surroundings with enthusiasm and uses safety seeking behaviour adequately when safe haven is needed), 2. insecure-avoidant attachment (the child shows low to no signs of separation anxiety in absence of primary caregiver, explores the environment less regardless of caregiver's absence or presence and does not seek comfort even in distressing situations) and 3. anxious-ambivalent attachment (the child shows high anxiety when separated from caregiver, is often too distressed and worried to explore their environment and seems to constantly seek protection from the caregiver). Mothers to avoidant children are often more cold and unaccepting towards negative emotions than mothers to secure children. Mothers to ambivalent children often show inconsistent patterns of how they attune to the child's emotional reactions. Researchers often use the "strange situation experiment" when evaluating attachment styles in children.