all about virus and antivirus

trojan horse or horse troya or more knowledgeable as trojan in computer security refer to a software form suspicious looking (malicious software/malware) that can botch a system or network. aim from trojan is to get information from target (password, recorded user habit in system logarithm, data,  and others),  and restrain target (gets access right in target).

procedure

trojan differs from software kind suspicious looking another likes computer virus or worm because two matters :
1. "stealth" (invisible and is not see)ed in the operation and oftentimes formed impressing program be program  well, temporary computer virus or worm act aggresiveer with botch system or make system is crash.
2.  trojan controlleds from other computer(computer attacker).

distribution manner

trojan horse meant to insert codes suspicious looking and botch in a program well and useful; just as in war troya, soldiers sparta hid in horses troya that attributed as dedication to poseidon.
majority trojan in this time shaped a bundle that can be executed (. exe or . com in windows operating system and Dos or program by the name of often executed in operating system unix, like ls, cat,  and others) that putted into system at penetrated by a cracker to steal important data for user (password, credit card data,  and others). also can infected when does user download application (the frequently of shaped game computer).

detection and cleaning

detect trojan existence is an action rather difficult done. line of least resistance with see port which opened and being in conditon" listening" , by using utility certain a kind of netstat. this matter is caused by many trojans walks as a system service,  and work at background, so that trojans can get command from assailant from long distance. 
last way is by using an antivirus software, equiped ability to detect trojan that merged with firewalls that monitor every transmission in and out. this way is efficienter, but more expensive, because usually antivirus software merged with firewall has price more expensive, for the alternative you can use a free antivirus that designed to remove trojan horse.