Kevin Mitnick

Social engineering is using manipulation, influence and deception to get a person, a trusted insider within an organization, to comply with a request, and the request is usually to release information or to perform some sort of action item that benefits that attacker.

Somebody could send you an office document or a PDF file, and as soon as you open it, it's a booby trap and the hacker has complete control of your computer. Another major problem is password management. People use the same password on multiple sites, so when the hacker compromises one site, they have your password for everywhere else.