The Unicode specification includes a huge number of different letters, symbols, characters, mathematical and musical symbols, dingbats, etc (known collectively as 'glyphs'). Each glyph is given a unique number and any piece of software that supports Unicode can, in theory, display any of the glyphs. In practice, this only happens if you have Unicode enabled fonts containing the relevant glyphs.
Unicode enabled fonts generally include only a subset of the total Unicode specification, for example Latin, Greek, Cyrillic and Thai, though there are a few fonts, such as Arial Unicode MS, which include a large proportion of the specification. Arial Unicode MS is included with Microsoft Office 2000.
Unicode specifies over 110,000 glyphs, and includes the following scripts:
Arabic, Armenian, Balinese, Bamum, Bassa Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid, Burmese, Canadian Aboriginal Syllabics, Caucasian Albanian, Cherokee, Chinese characters, Coptic, Cyrillic, Deseret, Devanagari, Duployan shorthand, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Hanunoo, Hiragana, Latin, Limbu, Mandaic, Malayalam, Mongolian, Hebrew, Kannada, Katakana, Kharosthi, Khojki, Khudawadi, Korean, Lao, Linear A, Linear B, Mahajani, Mandaic, Manichaean, Mende, Modi, Mro, Nabataean, New Tai Lue, N'Ko, Ogham, Old Italic, Old North Arabian, Old Permic, Old Persian, Oriya, Osmanya (Somali), Phags-pa, Pahawh Hmong, Palmyrene, Pau Cin Hau, Phoenician, Psalter Pahlavi, Runic, Shavian, Siiddham, Sinhala, Sumero-Akkadian Cuneiform, Syloti Nagri, Syriac, Tagbanwa, Tagalog, Tai Le, Tamil, Telugu, Thai, Thaana, Tibetan, Tifinagh, Tirhuta, Ugaritic, Varang Kshiti / Warang Citi, Yi
The Unicode specification also includes mathmatical symbols, musical notation and other assorted miscellaneous symbols.
Note: the Unicode specification is regularly up-dated, so new scripts may have been added since the above list was compiled. If you know of any additions that are not listed, please contact me.
You can use Unicode to add text in just about any language to webpages and even to mix languages/scripts on the same page. You can see examples of this on the following pages:
List of applications that support unicode
Information about character sets and internationalisation
Computing with Accents, Symbols & Foreign Scripts