In short, social engineering is the process of 'hacking' a human being.

Essentially, social engineering uses behaviors, language and other elements to elicit a response from the target - usually the act of unwittingly giving up information or allowing the attacker some kind of access to information or facilities.

It is frequently used by con artists, hackers and other less-than-savory people.

This is somewhat of a generalization, but that's a succinct description of the traditional form of social engineering.