The mouse in X works pretty much the same as the mouse on other
systems, except that it has three buttons. If your mouse has only
two, you can simulate the middle button by clicking both buttons
simultaneously. This is kind of tricky and annoying, so investing in
a $15 three-button mouse probably isn’t a bad idea. These are
available from most computer retailers.
The buttons are numbered from left to right assuming you have a
right-handed mouse. So button one is on the left, two is in the
middle, and three is on the right. You may see either the numbers or
the locations in documentation.
X has a simple built-in copy-and-paste facility. To select text to
copy, you click and drag with the left mouse button. This should
select the text to copy, assuming the application you’re using has
copy-and-paste support. To paste the text, you click the middle
mouse button in a different X application. For example, if you
receive an e-mail containing an URL, you can select the URL with the
left button and then click in your web browser’s “Location” field
with the middle button to paste it in.