Social engineering refers to manipulation of human psychology.
In the context of computer security, it refers to a non-technical breach of security that relies heavily on human interaction, i.e. tricking people into doing something that jeopardises their security or the security of the organization they work for.
Cybercriminals depend heavily on disguising malware and spam messages as legitimate communications, which may even pretend to be offering advice about how to combat cybercrime.
The objective is to get the victim to respond:
click on an infected e-mail attachment, click on a link to a compromised web site, or respond to a fake unsubscribe notice.