A trojan, or trojan horse, is a seemingly legitimate program which secretly performs other, usually malicious, functions. It is usually user-initiated and does not replicate.
Additional Details
Trojan:W32/Krap.B identifies software that has been packed with a particular Packer program known to be frequently used to compress malicious files.
The packer is custom made and has been designed to protect packed files from being debugged, emulated, or unpacked. The code in the packer stub is difficult to read and analyze since it has been obfuscated quite well. There are many decryption loops for the packer code itself which it executes during runtime and the code seems to be polymorphic.
The contents of the packed files are usually password stealers for onlinegames. They may also include some other types of malware such as trojans and worms.