Notepad3 is a Scintilla-based text editor with some Notepad2 code included, and a few additions of its own. These include code folding, support for bookmarks, the ption to mark all occurrences of a word, word auto-completion, syntax highlighting support and more.
If you don't recognise the names, it's essentially a smart plain text editor which you can use just like Notepad, but has plenty of useful extras just below the surface.
There's drag and drop editing, for instance (select a block, drag and drop it somewhere else); case conversion; bookmarking; powerful find and replacement with regular expression support; "always on top" and transparent window support; a zoom tool, extended clipboard, line manipulation tools, file change notifications, and more.
The program is aimed at developers, so you also get syntax highlighting, line numbers, code folding, visual brace matching and more.
If you're not a developer then you don't have to use, or even see most of these features. Click the View menu, clear "Line Numbers" and "Long Line Marker" and Notepad3 will look much like any other editor.