When copying text in OS X, you wind up with one of two things on the clipboard: formatted text, which retains font and style information, and unformatted text, which is simply the bare text. Which form you get is up to the program from which you're doing the copying—you don't have any say in the matter. Most of the time, this just works out. Sometimes, though, you want plain text instead of rich text. A good example is when copying text from Word and pasting it into iChat—when you paste into iChat, your pasted text is sent as an image, instead of text. Almost certainly, this is not what you intended.
But not all programs offer it, so what do you do if you run into one that doesn't? As a general solution, you can use an intermediary program that does—TextEdit, for instance. You could also paste it into a TextEdit document that was set to plain text mode (this will strip all formatting) and then paste it into your destination program.
Here's one more method—a specialized solution for an often-copied-from source program that doesn't require the use of any third-party applications. In Safari, copying portions of a web page retains all their formatting (and URLs are pasted as clickable links, too). If you don't want the formatting in your destination document, and it doesn't offer the Paste and Match Style option, here's a fast way to strip the formatting within Safari itself.
Select the text to be copied, copy it (Command-C), click once in the Google search box, then paste the copied text (Command-V). Even if you've selected a lot of text to copy, it will all paste successfully in the search box. Finally, select all in the search box (Command-A), copy that (Command-C), then switch to your destination program and paste (Command-V). If, like me, you prefer the keyboard, you can get to the Google search box in Safari by pressing Command-L (which selects the URL bar) then Tab. So the full sequence in Safari is: select text, Command-C, Command-L, Tab, Command-V, Command-A, Command-C. It looks much worse in writing than it is in actual use, and it definitely beats stopping at an intermediary application if your target app lacks the Paste and Match Style menu item. I sometimes use this method when the destination program has that menu item, simply because my fingers are so ingrained to type Command-V, not Command-Option-V.