Spear Phishing
In short: Spear phishing is targeted phishing against a specific person or organisation using personal details.
In more detail: The attackers research beforehand (social networks, company website) and write convincingly, for example in the name of the boss or a supplier.
In Depth
Whaling and CEO fraud
If the attack targets executives, it is called whaling. In CEO fraud a forged message from management asks accounting for an urgent transfer.
Protection
Four-eyes principle for payments, call back via a known number, share few private details publicly, know social engineering.
See also: Phishing, Social Engineering, E-mail Phishing