The most common
method used in the
X Window System for producing
characters which aren't directly assigned to a
key on the
keyboard uses a special key called the compose key. Formerly the
right Alt key was often designated the compose key, but the prevalence of
Windows keyboards and the relative uselessness of the extra keys outside of Windows has led to some combination of these keys often being assigned the role of compose key on
Linux systems.
To use this key, press it, then press two more keys (often in either order, and the "keys" may be combinations like shift-a to produce a capital A, shift-1 to produce a !, etc.). Various combinations of keys are defined to produce various symbols; for instance, compose-'-A produces Á. The combinations are intended to be mnemonic, so, for instance, all the letters with diacritical marks are produced by the combination of the letter and the ASCII character that most closely resembles the mark, and ligatures (like Æ) are produced by typing the two letters which are combined in that ligature.
The full list of compose key combinations can generally be found in the file /usr/X11R6/lib/X11/locale/iso8859-1/Compose (replace iso8859-1 with iso8859-2, etc., for other character sets in other locales).
The Windows world has a similar but different system, which uses the key binding Alt+number pad digits to type out the ASCII value of the character to be typed. This is much harder to use because it requires the user to know or look up the ASCII value of the character he wants to use.
Neither of these methods should be used for entering these characters on Everything; they are not valid HTML. Instead, use the HTML character entities as defined in E2 HTML tags.