Notepad++ is a free text editor.
There is a text editor that comes with Microsoft Windows, called Notepad. It doesn't do very much. Notepad++ is a far more intuitive replacement, that is better at things like Search and Replace, indenting lines, selecting whole words / lines / paragraphs etc.
Where it really comes into its own is with text documents that were really written for computers to understand: HTML, XML, Batch Files, .INI files, programming source code, JavaScript, CSS, you name it. It automatically applies intelligent colouring and highlighting to such files to make it much easier to see the structure of the file. All the standard conventions are used (such as comments being coloured green).
Highly recommended: I suggest you visit the homepage, then have a look at the screenshots, before going to the download page.