Block editing is the ability to edit, delete, insert text in a text editor with referece to verticle blocks as apose to lines. For example assume you have the follwing text.
<ul>
<li>Something</li>
<li>Something more</li>
<li>Another something</li>
</ul>
and you wanted to add a class to all three li tags (I know this is abit of a contrived example as you could just as easily add a class to the ul), ordinarily you'd have to add it to the first one and then copy and paste it into each other. Block editing allows you to select down vertically and then insert the same text on all the lines simultaneously(sp?). This can also be done for deleting, cutting, pasting etc. Although it doesn't sound like much once you get used to it you can't live without it.
