Just like almost every editing program ever written, you may use the mouse (or hotkeys) to select some content inside the editing window and either get rid of it (Cut) or just hold a copy in memory (Copy).  Either way, you may then insert the replicated selection at the cursor.

When a user elects to Copy content, this is usually saved to the system's clipboard.  Data stored in the clipboard may not only be pasted back into the application from which it was originally copied, but also to nearly any other application that can work with that type of data!  This is especially important for Contributing (and Core) Members.  It is best to save early and save often, this much is true, but it is also a very good thing to keep a copy for yourself!

It is best to compose offline.  Create your content in your preferred editor (don't worry about formatting it just yet) and, when it is finished, copy all of it to the clipboard.  Then you may begin creating a node of the appropriate type and simply paste in the full body.  All that remains is to add a bit of style (headers, etc.).

One may also, of course, elect to use the Paste from Word option but your mileage may vary.

Paste as plain text is probably the safest option, as it should completely strip all markup and other noise from the incoming content.

With the variety of options available, every user should hopefully find it relatively quick and easy to create even very large posts.