Keylogger

As the name suggest, this is a software that records the movements made by the user on their keyboard. Keylogging isn't necessarily criminal activity - it can be used by corportations and tech departments to troubleshoot technical problems.
However, it becomes a problem when malicious actors use this software to spy on their victim - by injecting a keylogger virus, they can steal PIN and bank account numbers, steal passwords, and infiltrate sensitive data. They can then pass all this information to a third party.
Addtionally, a keylogger can log the name of the user's webpage / active window - then, when a password is inserted by the user, the attacker can decipher which service that particular password belongs to.

There are two types of keylogger:
- Hardware: Hardware Devices can be embedded in the device itself
- Software: Phishing tactics can be used to implement the software, such as through email links and attachments

A keylogger will usually be banded together with other malware, such as adware, ransomware or Trojans.
The best way to detect if a keylogger has affected your device is to use an antivirus, which will regularly scan your device in order to see if there's been any unusual behavior.