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.
The latest version of Unicode (6.0) specifies nearly 110,000 glyphs. The following scripts are supported:
Arabic, Armenian, Balinese, Bamum, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid, Burmese, Canadian Aboriginal Syllabics, Cherokee, Chinese characters, Coptic, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Hanunoo, Hiragana, Latin, Limbu, Mandaic, Malayalam, Mongolian, Hebrew, Kannada, Kharosthi, Katakana, Korean, Lao, Mandaic, New Tai Lue, N'Ko, Ogham, Old Italic, Old Persian, Oriya, Osmanya (Somali), Phags-pa, Phoenician, Runic, Shavian, Sinhala, Sumero-Akkadian Cuneiform, Syloti Nagri, Syriac, Tagbanwa, Tagalog, Tai Le, Tamil, Telugu, Thai, Thaana, Tibetan, Tifinagh, Ugaritic, Yi
The Unicode specification also includes mathmatical symbols, musical notation and other assorted miscellaneous symbols.
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
Free Unicode text editor (includes Bitmap fonts for most of the scripts included in version 3.1 of Unicode): http://www.unipad.org
Free Unicode text editor for the X Window systems
Computing with Accents, Symbols & Foreign Scripts