It's a deterministic password-generator: put the same textual information (master password and website's name) in and get the same password out.
Using only a single Master Password that you need to remember and a keyword of your choice, you can use entirely different passwords for different websites!
Version 3.1 new feature: create portable version!
As it can store keywords (websites' names) and notes, it may be used as a password-organiser as well and, at a pinch, it could be a substitute/replacement for a password-manager.
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To Create a Portable Version:
Once installed, a portable version can be created by running the program and pressing the menu button and selecting the "Create portable version ..." option.
OR
Use the Universal Extractor program and extract the passgen.exe and passgen.chm files from the installer; put them in a folder (call it something like "Password Generator"); within that folder, create another folder and call this "profile" and, within that, create a blank text/ini file called "passgen3.ini".