Confused by CKEditor

I've read a little about CKEditor here but am still confused about what it is and how to use it. I can't figure out where it is to be used from.

Do you load it onto your computer, or do you upload it to your website? Is this a text editor that is used within or on a web page kind of like a CMS "comment" text box? If so, is it able to be used within an html environment? That is, on a strictly html static web page?

Sorry if this has all be covered before. I'm just trying to figure out how this is used.

Thank you, in advance, for any clarification about this.