A family of Microsoft word processing software products for creating web, email, and print documents.
Okay, so it's data that's been pasted in from a source other than a Word document. In that case it's probably caused by some formatting characters that Word just doesn't recognize.
For the most part Word does recognize current HTML formatting from text in websites but I would imagine that there might be some special formatting that it doesn't.
In any event I believe the best practice is to always Paste Special > Unformatted Text when brining in something from other sources, I prefer that method even between Word documents when I want to make sure my current document holds it styles pure and doesn't pickup extraneous ones.
With that said, if Paste Special is not working or Clear Formatting (and what I mean by "not working" is clearing extraneous formatting that you wish wasn't there) then you can also try first pasting the copied text into a Text Editor and then moving it from there into your Word document.
Hope this helps