Trojans are a common type of malware, which, unlike viruses, can't spread on their own. This means they either have to be downloaded manually or another malware needs to download and install them.
Trojans often use the same file names as real and legitimate apps. It's easy to accidentally download a trojan thinking that it's a legitimate app.
How trojans work
Trojans can come in many different varieties, but generally they do the following tasks:
Download and install other malware, such as viruses or worms.
Use the infected device for select fraud.
Record keystrokes and websites visited.
Send information about the infected device to a malicious hacker including passwords, sign in details for websites, and browsing history.
Give a malicious hacker control over the infected device.
How to protect against trojans
Use the following free Microsoft software to detect and remove it:
For more general tips, see prevent malware infection.