A font typically defines shapes for a single alphabet or script. Therefore, displaying the entire range of scripts that Emacs supports requires a collection of many fonts. In Emacs, such a collection is called a fontset. A fontset is defined by a list of font specs, each assigned to handle a range of character codes, and may fall back on another fontset for characters which are not covered by the fonts it specifies.
Each fontset has a name, like a font. However, while fonts are stored in the system and the available font names are defined by the system, fontsets are defined within Emacs itself. Once you have defined a fontset, you can use it within Emacs by specifying its name, anywhere that you could use a single font. Of course, Emacs fontsets can use only the fonts that the system supports; if certain characters appear on the screen as hollow boxes, this means that the fontset in use for them has no font for those characters.1
Emacs creates three fontsets automatically: the standard fontset, the startup fontset and the default fontset. The default fontset is most likely to have fonts for a wide variety of non-ASCII characters and is the default fallback for the other two fontsets, and if you set a default font rather than fontset. However it does not specify font family names, so results can be somewhat random if you use it directly. You can specify use of a specific fontset with the ‘-fn’ option. For example,
emacs -fn fontset-standard
You can also specify a fontset with the ‘Font’ resource (see X Resources).
If no fontset is specified for use, then Emacs uses an ASCII font, with ‘fontset-default’ as a fallback for characters the font does not cover. The standard fontset is only used if explicitly requested, despite its name.
A fontset does not necessarily specify a font for every character code. If a fontset specifies no font for a certain character, or if it specifies a font that does not exist on your system, then it cannot display that character properly. It will display that character as an empty box instead.
 The Emacs installation instructions have information on additional font support.