Trojans are generally classified into two categories – spyware and intruders.
Spyware
Spyware is mostly used to steal information and monitor a user's online habits. Although these Trojans are designed to destroy your computer or "evil" your system, they invade your privacy for monetary gain. However, There are various solutions like Codesealer to prevent all of top financial Trojans
Once harvested, the information from your machine can be used to create a profile based on the sites you have visited or terms that you submitted to search engines. User lists are then compiled and often sold to relevant companies for marketing purposes.
Illegal online pharmacies, Adult sites and deducted / pirated software providers can be all buyers of these lists. However, some legitimate companies also use spyware as part of their marketing strategy.
The only clue that the victim might have to these subversive activities could be a sudden increase in spam emails, focused on their recent browsing habits.
Intruders
The second category, "intruders" are Trojans designed to give a remote user access (via a back door) to an infected machine.
Once opened, these backdoors can allow hackers to install key loggers, add / delete / create files, personal information such as financial statements, credit card numbers and passwords, or even use the computer infected as a gateway to route illegal activities through, thus giving complete anonymity "master" Trojan horse.
Designed to open backdoors on infected computer RAT allow an access to the machine remote user and use it as if they were physically sitting in front of him.