A recent class of threats, known as Advanced Persistent Threats (APTs), has drawn increasing attention from researchers, primarily from the industrial security sector. APTs are cyber attacks executed by sophisticated and well-resourced adversaries targeting specific information in high-profile companies and governments, usually in a long term campaign involving different steps. To a significant extent, the academic community has neglected the specificity of these threats and as such an objective approach to the APT issue is lacking. In this paper, we present the results of a comprehensive study on APT, characterizing its distinguishing characteristics and attack model, and analyzing techniques commonly seen in APT attacks. We also enumerate some non-conventional countermeasures that can help to mitigate APTs, hereby highlighting the directions for future research.