Overview
The following definition for social engineering comes from the U.S. Department of Homeland Security:
In a social engineering attack, an attacker uses human interaction (social skills) to obtain or compromise information about an organization or its computer systems. An attacker may seem unassuming and respectable, possibly claiming to be a new employee, repair person, or researcher and even offering credentials to support that identity. However, by asking questions, he or she may be able to piece together enough information to infiltrate an organization's network. If an attacker is not able to gather enough information from one source, he or she may contact another source within the same organization and rely on the information from the first source to add to his or her credibility.
Social engineering is a broad field that includes not just phishing and spamming, but also actual face-to-face interactions between hackers and their unsuspecting victims.
This page explains some of the simple things you can do to protect yourself from these types of attacks.