How is cut, copy and paste done in X?

In general, all X applications understand two mouse sequences for copy and paste. Not all applications understand cut, and nor is it appropriate to cut in all applications.

Copy is ordinary drag-selecting with the left button. Paste is the middle button (on two button mice, press both buttons at once), and for those applications that understand it, double right click will cut.

Another way of pasting stuff is using the keyboard: [shift][ins] will usually work.