Trojan.MacOS.Downloader.Z is a generic detection of a downloader component of a trojan infection. Downloaders are the components of threats which are designed to download the payload or additional components.
Trojan is short for trojan horse and the term is derived from an ancient greek myth. Threats that are designated as trojans can serve many purposes but have one thing in common. The feature that defines trojans is stealth. Trojans aim to infiltrate a system and carry out their functions without the knowledge of the victim. For that purpose, trojans are often included as hidden or hard-to-find components of freeware bundles. In other cases, trojans can sneak in disguised as fake updates, most commonly Adobe Flash Player updates.
Once a trojan has infected a system, there are numerous tasks it can perform. Trojans often serve as intermediary infections, meaning they infect a device to facilitate the download and installation of other more sophisticated threats. Another common use for trojans is data collection and exfiltration. The information a trojan can gather may vary from threat to threat. Cybercriminals often go after banking credentials or credentials for other accounts which can ensure further points of entry.
Developers of security solutions strive to stay on top of new definitions for all threats including trojans. This allows the timely detection of new threats as new trojans and other malware are developed daily.