Within the manager interface Piranha uses a WYSISYG editor for editing formatted HTML content. You can read more about how to add a custom HTML editor to your Manager instance on the HTML Editors page under Manager Extensions.
At the moment Piranha has packages for two different editors, TinyMCE and Summernote. Tiny MCE has been the de facto standard for many years and is a very powerful editor. Summernote is a newer and more lightweight alternative. Which editor you choose depends on personal preference, but also on licensing and other factors.
As we don't have time to keep track of how the providers of these external components might change their licensing options over time, please always double check the official site so that their current licensing terms matches your application.