HTML editors can be something as simple as Notepad which allows you to type your html code by hand, into plain text files, and save them with the .html extension.
Another type of HTML editor is the "WYSIWYG" editor. This is a pneumonic for "What You See Is What You Get". These programs allow you to create websites quickly without prior knowledge of HTML. They are usually template based with several drag and drop elements. Examples of a WYSIWYG editor include DreamWeaver and MS FrontPage.
One down site of a WYSIWYG html editor is that they product somewhat bloated code and you don't have the complete control that you do with a text based html editor.
Usually the knowledge of the HTML language is what determines what type of editor a person perfers. Novices might prefer the WYSIWYG editor while most experienced webmasters will chose a text based html editor for more control.