By default when you paste Word will paste with formatting. Sometimes you don't want to do that, you want to paste just the text (no formatting) or you want to merge the formatting of what you are pasting with what you already have in your document. For me I would say that pasting as text is the most useful as it means after pasting I can decide what format I would like to apply, that's what I feel more comfortable with. That's the key really, in Word you can control how pasting works in different ways. I'll show a few of these but it's up to you ultimately how you work and which makes sense to you.
On the Home menu there's a Paste button. It has a number of options, 'Keep Source Formatting', 'Merge Formatting' and 'Keep Text Only'. In the example below I've written "Mary had a little lamb..." and I've added some formatting, I've copied this and one by one I'll show the three paste possibilities:
Keep Source Formatting:
The first option means that when you paste the text and the formatting will be included. This is the default option which means it's highlighted here initially and also when you press Ctrl-V this is what will happen (this can be changed, keep reading to find out how!).
Keep Text Only:
This one can be very useful, it allows you to paste just the text, with no formatting. If you will decide on the formatting later, this could be the best option.
This adds the formatting included in the paste with what's already in your document. Personally I don't use this very often.
Right Click Paste
If you click the right mouse button you'll see the paste options on the context menu. The options are exactly the same as explained above.
Position the cursor where you want the text to go and press Ctrl-V. The default paste option will be used. This is 'Keep Source Formatting' unless you've changed the default (explained later in this article).
Paste using the Smart Tag
When you press Ctrl-V a Smart Tag will appear, click it and you have the same options as explained above.
Change the Default Paste option
Click File | Options | Advanced
Scroll down a little, you'll see the 'Cut, copy and paste' section:
Next to 'Pasting within the same document' you can change Keep Source Formatting to Keep Text Only or Merge. Click OK.
Both Word 2013 and 2010.