Text which has been copied to the clipboard via Ctrl-c (or cut with Ctrl-x) can be pasted directly into an rxvt terminal by positioning the cursor in the rxvt terminal where you want the text inserted and holding the shift key while clicking the left mouse button.
To copy text from an rxvt terminal simply drag the cursor over the region of text you want to select with the mouse while holding the left button down. Selected text will be highlighted and available in the clipboard for pasting into other applications via Ctrl-v. Double-clicking on a word will select the entire word. Clicking three times will selected an entire line. To select a range of words you can double click on the first word, then set the mouse cursor to the right of the last word or letter you want copied, and right-click.