While those of us who live in anglophone countries are blessed with having our characterset be the default for modern computing; there are plenty of others not so lucky and while we have keyboard layouts which allow many of them to type letters such as é, ç, ø, and ł, for those of us who do not have those characters on our keyboards there are a number of ways for us to enter them.

If you use a GTK+ based system then the default input method supports direct entry of Unicode codepoints by means of holding Control, Shift, pressing u and then typing the codepoint in, in hexadecimal before releasing the chording keys. For example, C-S-u 266b produces ♫. In addition, if you're using an application which supports the X11 Compose key, then there are composition sequences for many characters which can be accessed by pressing the Compose key and then the sequence of characters which comprise the composition sequence. For example, Compose C = produces €.

In addition to composition sequences, some keyboard layouts support what are called dead keys, sometimes in alternative shift levels on the keyboard accessed via a level shifting key. A keyboard with a dead ' can produce an é by means of pressing ' and then e.

Finally, if you want to enter other kinds of characters such as 한국어 then you will need a more complex input method. There exist a number but the more commonly encountered ones are uim and fcitx. There are plenty of tutorials for setting up uim or fcitx (or one of the others) in your desktop environment if you search the interwebs. These input methods are special because they often require the ability to enter incomplete characters to prompt you for further input and as such they break the basic rule of one keypress produces one character (though that was already bent with the compose and dead keys).

Your homework is to delve into the keyboard settings on your system, find out where your compose key is, and play with composition sequences (you can find examples in /usr/share/X11/locale/en_US.UTF-8/Compose or a similar location depending on your chosen locale). The composition sequences often also list the dead key combinations so have a good explore and learn how to type all sorts of characters you might previously have gone to a character map application for.