Keyloggers have been well known for a long time. They can monitor keystrokes and log them to a file. The log file can be transferred to the hacker later on.

A password stealer is a similar thing. It can steal usernames and passwords from the following locations:

• Browsers store passwords for social networking sites, movie sites, song sites, email, and gaming sites.
• FTP clients such as filezilla and smartftp, which can be used in companies or individuals to save data in FTP servers.
• Email clients such as Thunderbird and Outlook are used to access emails easily.
• Database clients used mostly by engineers and students
• Banking applications
• Users store passwords in password managers so that they don't have to remember them. Malware can steal passwords from these applications. LastPass and KeepPass are password manager applications.

Hackers can use these credentials to steal more data or access the private information of somebody or to try to access military installations. They can target executives using this kind of malware to steal their confidential information.

zeus and citadel are famous password stealers.