Here is description of the most widespread kinds of trojan applications.

Mail Senders
This is the most widespread group. The most of them send passwords to their owners. Here are some examples: Trojan-PSW.Win32.QQPass.du (Chinese trojan, steals Windows passwords), Bandra.BOK (it is downloaded during visit of specific site and steals passwords of bank account), Bancos.LU (saves passwords as temporary files and sends them to remote owner), Banker.XP (collects personal information, passwords, accounts and sends them to customized address).

Remote control applications
Such applications usually have opportunities of mail senders in combination with functions of remote computer control. Trojans of this type wait for connection on the client side and send commands to server. Examples: Backdor.Win32.Whisper.a – is a trojan which gives an opportunity to control remote computer. Famous Back Office which was created by Cult of Dead Cow hackers group. This application gives an opportunity to control remote computer, to access all hard drives, to get online picture of screen, to hear and to look after user by means of microphone and web camera.

Dialers
This kind of applications is not widespread today because of spreading of more present day technology but it is still dangerous. Such application can cause financial expenditure. Dealers change provider’s phone number and make a call to exotic countries. Examples: Trojan- PSW.Win32.DUT, Trojan-PSW.Win32.Delf.gj, not-a-virus:PSWTool.Win32.DialUpPaper, not-a-virus:PornWare.Dialer.RTSMini.

Keyloggers
This kind of applications also includes functions of mail senders. They are able to track keystrokes and send this information to malevolent user. Examples: Backdoor.Win32.Assasin.20, Backdoor.Win32.Assasin.20.n, Backdoor.Win32.BadBoy, Backdoor.Win32.Bancodor.d (keylogger.trojan).

DDos emulators (Distributed Denial of Service)
It is very interesting type of trojan applications. Server side application is waiting for a signal and after getting it begins working as nuker (such applications send gust of data packages to customized IP)

Downloaders
This trojans download files from global network without user’s knowledge. This files can be either adult pages or harmful software. Example: Trojan-Downloader.Win32.Agent.fk (It creates %Program Files%\Archive folder and copies itself to it)

Droppers
The aim of such applications is concealed installation of other trojans. Example: Trojan-Dropper.Win32.Agent.vw.