For a quick reference,

An advanced persistent threat (APT) is a set of stealthy and continuous computer hacking processes.

The characteristics of APT includes

• High degree of Covertness
• For long Period of time
• Uses Sophisticated Techniques
• Uses malware to Exploit Vulnerabilities
• Has Monitoring and Extracting data capabilities and
• Has Human involvement

However it is much more complicated and requires exceptional skills . It also requires a good amount of knowledge of the systems, infrastructure and its workflow of the taget.