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Unicode is a system for representing characters numerically, and is the basis for various character encodings including the popular UTF-8. It was devised beginning in 1987, with the first version of its standard published starting in 1991. Subsequent revisions have continually expanded its character repertoire.
Unicode was devised in reaction both to the unwieldy multiplicity of character sets that had arisen to include various subsets of the many characters left out of the English-centric ASCII set, and to the clumsiness of the proposed ISO 10646 character set which was the awkward product of international politics rather than a sensible technical system. It has been successful to the point where just about all technical standards dealing with characters now are defined with regard to Unicode code points, with even the older proprietary encodings cross-referenced to the Unicode characters they encode.
Early versions of Unicode attempted to be a 16-bit character encoding where characters in a potential repertoire of 65,536 code points could be represented as 16-bit (2-byte) unsigned integers, though the "big-endian vs. little-endian" problem caused there to be two possible byte streams corresponding to a particular document, which could be disambiguated using the "byte-order marker" character which came out as (hexadecimal) FFFE or FEFF depending on which byte order was used. However, from an early time, users of Unicode preferred encodings which did not take up two bytes for every character, even common ones from the ASCII repertoire, so variable-byte-length encodings came into common use. Ultimately, the Unicode standard expanded its repertoire using multiple "planes" so that even 16 bits weren't enough to encode all possible characters.
At its root, Unicode is simply an assignment of numeric values to characters, where a huge number of characters from various writing systems (modern or ancient) as well as special symbols of many types are each given a number. Usually in Unicode charts these are given as hexadecimal numbers rather than the decimal ones most humans tend to prefer (computer geeks are funny that way), but they can be expressed in any base you want; in HTML you can use ampersand encoding with either decimal or hexadecimal numbers to express a Unicode character (e.g., A or A).
Once these numbers are assigned, they can be encoded as sequences of bytes in various ways, defined in the specifications of particular character encodings.
The first 128 Unicode code points, 0-127, correspond to the same code points in ASCII. The next 128, 128-255, correspond to the same points in ISO-8859-1, which in turn contains the same characters at 0-127 as ASCII, so the entire first 256 characters in Unicode are equivalent to that standard. Since UTF-8 encodes characters 0-127 as raw bytes of the appropriate number, a file consisting entirely of ASCII characters will be identical in the us-ascii and UTF-8 encodings.
- Unicode official site -- has lots of standards documents and code charts
- Wikipedia entry on Unicode
- Wikipedia list of Unicode Characters
- Shapecatcher - Site for finding unicode characters by drawing them
- http://rishida.net/tools/conversion/ - Tool for converting Unicode into other character formats such as UTF-8, HTML, etc.
- http://www.fileformat.info/info/unicode/char/search.htm - Unicode search at Fileformat.Info