Inside an xterm, if Button 1 of the mouse is held down and dragged, the text is highlighted. Pressing Button 2 while the text is still highlighted causes the text to be pasted into the window. This is a handy way of transferring text from one xterm to another.
Inside an xterm, each mouse button is also associated with a menu. If you press Ctrl-Button 1 you will get the Main Options menu, which can be useful if the program you are running in that xterm runs into trouble or you want to get rid of that xterm. Ctrl-Button 2 brings up the VT Options menu, which controls various ways the xterm behaves such as line wrapping and scroll bars. The third menu, appearing when you press Ctrl-Button 3, is the VT Fonts menu, which allows you to change the font size of that xterm.
These menus are available independent of the window manager you are using.