What is an Advanced Persistent Threat (APT)? What is the goal of an APT? How can a business protect itself from an APT? What tools can be used to detect

What is an Advanced Persistent Threat (APT)? What is the goal of an APT? How can a business protect itself from an APT? What tools can be used to detect an APT? Provide links to sources or sites that explain these tools in more detail.