At Microsoft we define social engineering as involving clever manipulation of the natural human tendencies of trust in order to obtain information to help facilitate fraud, network intrusion, industrial espionage, identity theft, or network/system disruption. I also like the definition from Bruce Schneider: Amateurs Hack Systems, Professional hack People.

To gain the trust of the people social engineers trick their victims with different tactics such as:

• Pretending to be someone important
• Appearing to be just like you
• Trying to convince you to share confidential information