A woman's attachment style determines whether she is clingy, or distant, or prone to upset at the most trivial thing. Know her style, and you know what to expect.
Early in life, the way someone's parents raise her shapes the way her brain deals with her relationships with other people.
If her parents are loving and supportive, and around enough, and not abusive or neglectful, she'll form a secure attachment style.
Secure attachment is the healthiest form of attachment: it's the ability to form real bonds with other people, free from either neediness or (on the other hand) an urge to 'get away'.
However, not all parents are loving and supportive, or present enough; some are neglectful, or even abusive. Some parents have attachment disorders themselves and simply aren't good at parenting. And when a child grows up with parents like this, she learns different lessons about close relationships, and her brain wires itself to respond differently to closeness.
Depending on her upbringing, she may learn to cling to others for dear life. She may flee closeness, and try to keep space in her relationships. She may have chaotic emotions toward closeness and vacillate between wanting it and pushing it away.
Once you know attachment styles, you can better grasp the women you bring into your life... and better grasp yourself.