A Trojan Dropper is a type of Trojan whose purpose is to deliver an enclosed payload onto a destination host computer. A dropper is a means to an end rather than the end itself. In other words, the dropper is usually used at the start or in the early stages of a malware attack. Once a dropper is executed, its own code is simply to load itself into memory and then extract the malware payload and write it to the file system. It may perform any installation procedures and execute the newly dropped malware. The dropper usually ceases to execute at this point as its primary function has been accomplished. Trojan-Downloaders are another type of Trojan that are often distributed as part of the payload of a Trojan Dropper.

Recent trends show a downloader is used to grab a final payload and also installs the final payload. So the dropper puts the virus on a system and the downloader beacons out to pull down the ultimate payload. EDIT: Reworded the questions and answers to match https://blog.malwarebytes.com/threats/trojan-dropper/