It is a common practice of almost everybody who works with Tridion: copying text from Word or Excel into a rich text field in Tridion. Unfortunately, when you do this you end up copying much more than just the text. Styling characteristics also find their way into the rich text field, in the form a style attributes, class attributes, comments, unnecessary spans, and even complete sets of CSS classes. And when a component with such a rich text field is published, all this styling finds its way to your site, where it is very likely to clash with your carefully designed pages.
Of course, you can circumnavigate this issue by copying first to a text editor like Notepad, copy it again and then pasting it into Tridion. But there is a more elegant solution for this problem. It requires just a tiny bit of work.