A trojan-downloader is a type of trojan that installs itself to the system and waits until an Internet connection becomes available to connect to a remote server or website in order to download additional programs (usually malware) onto the infected computer.
Attackers often distribute trojan-downloaders as part of the payload of another harmful program, such as a trojan-dropper, which silently drop and install the trojan-downloader on a device.
Trojan-downloaders are also commonly distributed as disguised file attached to spam emails. The attached programs are typically labelled using legitimate-sounding program or document names, such as 'invoice' or 'accounts.exe', as a simple form of social engineering. If the file attachment is opened, the trojan-downloader is installed.
Once a trojan-downloader has been installed on a machine, it will try to contact to a remove server or website, where it can either directly fetch additional files for download, or find further instructions from the attackers on where to find the files.
For representative examples of trojan-downloaders, please see the following descriptions: