Most applications have an Edit menu with menu items such as "Cut", "Copy" and "Paste". However, older X applications may not have these menus, or they may not work correctly. The procedure described below should work for all X applications.
(NOTE: Emacs behaves slightly differently. See How do I use the mouse buttons in Emacs? for more information.)
Move the mouse to one end of the section of text. Press down the left mouse button, and while holding it down, drag the mouse to the other end of the section. Release the button, and the text will remain highlighted.
Move the mouse to one end of the section of text. Click the left mouse button once. Then move the mouse to the other end of the section and click the right mouse button once.
Click the left mouse button twice on the word.
Click the left mouse button three times on the line.
Click the middle mouse button.