Sometimes you gotta paste text in so that it looks exactly like the original you copied. Sometimes, though, you want it to match the formatting you've got going on in the rest of your document, and it can be really really REALLY time-consuming to go through and reformat all the pasted text you've inserted. Enter Apple's lovely solution to that issue—Edit > Paste and Match Style (or Option-Shift-Command-V).
You can use this feature in a lot of different programs around your Mac, like Mail, Pages, Notes, and even Word (it's labeled Edit > Paste and Match Formatting in Word 2011).
Here's what effect it has. I copied the headers for a TMO article, and it looks like this if I use the regular Edit > Paste (Command-V) menu item while I'm composing a message in Apple Mail:
And here's how it looks using Edit > Paste and Match Style:
Paste and Match Style forced the text to conform to my regular Mail font, size, and color, and it stripped out the extraneous HTML (such as the link to the article that got copied with the headline). Neat, right? This is obviously pretty useful when you're pulling information from different sources but want to make everything look tidy.
As a bonus tip, here's another way to paste text in Apple Mail that I also use quite often. It's Edit > Paste as Quotation (Shift-Command-V). Here's what it does to our headline:
As you can see, it sets it off from the text I typed around it by increasing what's called the Quote Level of the pasted item, indicated by the blue bar to the left of that text.
If you want to change that Quote Level manually, you can select some text and choose Format > Quote Level > Increase, which can be applied multiple times to make the level increase even further. Use this to really make your emails look professional when you're quoting different sources.
What? I said you could make your emails look professional. I didn't say anything about mine.