This article is from the Computer Viruses FAQ, by Nick FitzGerald firstname.lastname@example.org with numerous contributions by others.
A DROPPER is a program that has been designed or modified to "install" a
virus onto the target system. The virus code is usually contained in a
dropper in such a way that it won't be detected by virus scanners that
normally detect that virus (i.e., the dropper program is not *infected*
with the virus). While quite uncommon, a few droppers have been
discovered. A dropper is effectively a Trojan Horse (see B3) whose
payload is installing a virus infection. A dropper which installs a
virus only in memory (without infecting anything on the disk) is
sometimes called an "injector".