I finally learned the difference between highlighting+middle mouse clicking (or SHIFT+INSERT if you’re on the keyboard) and copying/pasting text in Linux today, all thanks to the manpage for the nifty xsel program:
The X server maintains three selections, called PRIMARY, SECONDARY and CLIPBOARD. The PRIMARY selection is conventionally used to implement copying and pasting via the middle mouse button. The SECONDARY and CLIPBOARD selections are less frequently used by application programs.
There is no X selection buffer. The selection mechanism in X11 is an interclient communication mediated by the X server each time any program wishes to know the selection contents, eg. to perform a middle mouse button paste.
That is the most useful explanation about highlighted text vs. copied text I’ve seen since entering the Linux world a few years ago. The PRIMARY selection (highlight + middle mouse click) is, as far as I can tell, available in pretty much every Linux program. I’ve never encountered the SECONDARY selection type. The CLIPBOARD selection is somewhat rare; I’ve only encountered it when I explicitly highlight some text in Firefox and then do right-click -> Copy (before I knew about PRIMARY selections).
As for xsel itself, you can use it as a very simple buffer. The best use scenario would be a case where you want to use the output of some command for a while, but where creating a temporary file would be overkill. Here is a random example of xsel:
$ <file.txt | tail -n1 | xsel -p
Now the last line of file.txt is in your PRIMARY selection buffer. Then, after some other miscellaneous work in your shell, you can recall what’s in the PRIMARY buffer like so:
$ xsel | less
The -p flag makes the piped text go into the PRIMARY selection buffer. However, the -p flag is the default one, so you can omit it. Interestingly, the -x flag can be given to swap the PRIMARY and SECONDARY selections.