On it’s surface, Unicode is simple. It’s a replacement for ASCII to make room for more characters. Joel Spolsky assures us that it’s not that hard. But then how did Jukka Korpela have enough to say to fill his 678-page book Unicode Explained? Why is the Unicode standard 1472 printed pages?
It’s hard to say anything pithy about Unicode that is entirely correct. The best way to approach Unicode may be through a sequence of partially true statements.
The first approximation to a description of Unicode is that it is a 16 bit character set. Sixteen bits are enough to represent the union of all previous character set standards. It’s enough to contain nearly 30,000 CJK (Chinese-Japanese-Korean) characters with space left for mathematical symbols, braille, dingbats, etc.
Actually, Unicode is a 32-bit character set. It started out as a 16-bit character set. The first 16 bit range of the Unicode standard is called the Basic Multilingual Plane (BMP), and is complete for most purposes. The regions outside the BMP contain characters for archaic and fictional languages, rare CJK characters, and various symbols.
So essentially Unicode is just a catalog of characters with each character assigned a number and a standard name. What could be so complicated about that?
Well, for starters there’s the issue of just what constitutes a character. For example, Greek writes the letter sigma as σ in the middle of a word but as ς at the end of a word. Are σ and ς two representations of one character or two characters? (Unicode says two characters.) Should the Greek letter π and the mathematical constant π be the same character? (Unicode says yes.) Should the Greek letter Ω and the symbol for electrical resistance in Ohms Ω be the same character? (Unicode says no.) The difficulties get more subtle (and politically charged) when considering Asian ideographs.
Once have agreement on how to catalog tens of thousands of characters, there’s still the question of how to map the Unicode characters to bytes. You could think of each byte representation as a compression or compatibility scheme. The most commonly used systems are UTF-8, and UTF-16. The former is more compact (for Western languages) and compatible with ASCII. The latter is simpler to process. Once you agree on a byte representation, there’s the issue of how to order the bytes (endianness).
Once you’ve resolved character sets and encoding, there remain issues of software compatibility. For example, which web browsers and operating systems support which representations of Unicode? Which operating systems supply fonts for which characters? How do they behave when the desired font is unavailable? How do various programming languages support Unicode? What software can be used to produce Unicode? What happens when you copy a Unicode string from one program and paste it into another?
Things get even more complicated when you want to process Unicode text because this brings up internationalization and localization issues. These are extremely complex, though they’re not complexities with Unicode per se.
For more links, see my Unicode resources.