a basic highlighting editor?

One that will highlight the syntax of your HTML so that it is easier to pick out tags. Usually, they will highlight different tags in different colors/font styles, and they will often highlight the text that has been marked up, as well (like making text surrounded by <B> tags bold and <A> tags underlined, etc). They're useful because it's easier to see your layout and they usually also perform some very basic syntax checking.

What's HTML Tidy?

It's a program that tidies your HTML, formatting it in a more consistent fashion, and sometimes editing out extraneous tags, especially the stuff MS products like to put in. HTML Tidy