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 drops and installs the trojan-downloader on a device.
They may also be distributed as disguised files attached to emails. In these cases, the attackers rely on social engineering to trick users into opening the attached files, which then silently installs the trojan-downloader.
Once a trojan-downloader has been installed on a machine, it will try to contact 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.