Trojans are malicious applications that pose themselves as legitimate software in order to trick users to install them. Once on the victim's machine, it may run any number of malicious process to steal vital information or inflict damage to other software.
Basically, Poly-DL is a semi polymorphic web downloader that is very small. Its based on several different stubs, each having a different ammount of empty space throughout them. When the server is made, that space is filled in with random characters then packed, thus producing a different server every time and a psudo polymorphic affect. There is the option to mess up the packed header and add junk bytes at the end of the file too. To create a greater polymorphic affect, select the 15kb option, 10kb for a lesser affect, ect. 0 Bytes produces the smallest server (1kb!) but has the least ammount of randomness.'