This application is a text editor for the Microsoft Windows operating system that is designed with writing programming code in mind. It merges the simplicity of Microsoft's own Notepad application, with additional features that many users of Notepad find useful, such as syntax highlighting, and ability to open multiple documents.
Notepad++ is an established application that has been around for a long time, and as such, is trusted. However, it is available for download from a number of locations, and this introduces the risk of accidentally downloading malicious software. You should always try to download Notepad++ from the official site.
The Notepad++ application is not, by itself, a compiler. This means that, while you can use the application to write code, you cannot use it to test the code. With the help of external software, such as MinGW, Notepad++ can be configured to compile and execute C and C++ code.
Notepad++ is indeed and open source application. Open source means that, not only is the application free, but the source code for it is also freely available for anyone to download, and modify as they see fit. In this way, the application can be maintained by many people/organisations at once.
Unfortunately, Notepad++ is not available on the OSX or other Apple operating systems. Initially inspired by Microsoft's Notepad application, Notepad++ is a Windows application only. There are, however, a number of alternatives for both Mac and Linux operating systems, not to mention a range of alternatives on the Windows platform.
The application does not have a spellchecking feature. It does support plugins, however, and this feature can be added through those means. A spellcheck plugin was included in the base installation for a time, but has since been removed. There is nothing to stop the plugin being manually added, however.
Notepad Plus supports Java in terms of syntax checking. This means that, as you write Java code, the app highlights the code, making it easier to understand, and also to see potential errors. It cannot execute Java code, however it can be used with the Java JDK to test code.
First you will need to download the installer. Do this from the official Notepad++ site to avoid potentially downloading malicious software. For similar reasons it's always best to get the latest version, as old versions may contain security flaws. Once downloaded, simply run the installer and follow the onscreen instructions.
Notepad++ comes with a program for checking for updates. It can be run entirely separately, or you can set Notepad++ to run it on launch in the app's settings. Alternatively, you can uninstall your current version of the application manually, and download the latest one and install it as normal.
Notepad++ is free. It is an open source application and, as such there is no fee for downloading or using it. There is an option to make a donation on the app's homepage. The developers of the app are essentially working for free, so donations help keep the app supported.