man xclip says:
-selection specify which X selection to use, options are "primary" to use XA_PRIMARY (default), "secondary" for XA_SECONDARY or "clipboard" for XA_CLIPBOARD
Why do we have 3 types of X-Selections? What are their purposes?
Historical partly. I'm not sure secondary is used for anything at all anymore.
"cliboard" is what is used by most ctrl+x/c/v, it is the clipboard you put things into on purpose
"primary" is what holds whatever text is currently selected, it is how X keeps track of what is selected and conveniently may be pasted by middle clicking (or, if you're awesome, clicking right and left buttons at the same time) or by shift+insert in xterm