What are the attachment styles in romantic relationships?

According to the attachment theory, adults are characterised, in their romantic relationships, by one of three styles.
These are:

• Secure lovers
  People who find it easy to get close to others and are comfortable having others feel close to them
  Mutual dependency in a relationship feels right
  They do not fear abandonment
• Fearful or avoidant lovers
  Uncomfortable feeling close to another person or having that person feel close to them
  It is difficult for them to trust or depend on a partner
• Preocupied or anxious-ambivalent lovers
  Want desperately to get close to a partner but often find the partner does not reciprocate the feelings
  They are insecure in the relationship, worrying that the partner does not really love them