Notepad++ is a feature-full text editor.
By default, Notepad++ remembers the documents you've opened across sessions, and requires you to manually close them or they will continue to reopen with each subsequent use. I do not like this feature, but it isn't obvious in the settings where to turn it off. There is a section called Recent File History, but this only applies to recent files listed in the File menu. To prevent Notepad++ from keeping files open across sessions, go into Settings -> Preferences -> Backup, and uncheck "Remember current session for next launch."