Chinese is a symbolic language that uses characters. The Chinese language has two written versions: Traditional and Simplified. Traditional Chinese characters are complex because they contain more brush strokes per character, and are mainly used in Hong Kong, Macau, Taiwan and various other areas in the English speaking world. Simplified Chinese has been "simplified" by using less brush strokes per character, and are mostly used in Mainland China and Singapore.

Although simplified characters have been increasing in popularity, traditional Chinese characters are still prevalent.  A great number of publications use simplified characters, but there are many works available only in traditional Chinese character format.  

Computers use numbers to map Chinese characters. Special mappings between numbers and letters or characters, are made into standards that are interpreted by various computers and programs. These agreed upon ways of using Chinese are called character sets or code sets. 

For Simplified Chinese, GB (short for Guojia Biaozhun or "National Standard") is the standard, which is used in the People's Republic of China and Singapore. It has a set of about 7,000 simplified Chinese characters. 

For traditional Chinese, the standard is Big5. It is used in Taiwan and Hong Kong and has about 13,000 traditional Chinese characters. 

Unicode is an emerging standard that attempts to encode all major languages, including Chinese. Unicode includes all the characters from GB and Big5 and many more. A character set is different from a font that supports that character set. For instance, you may have a document written in GB, but to view it you need a font that includes all GB characters. Viewing a GB encoded document  in Big5 will produce litter on the screen. Viewing a Chinese document on a program that thinks it is in English will also produce an unintelligible document with lots of accented letters and symbols.

The Chinese characters in Unicode are a superset of the characters in GB and Big5, so it is easy to convert directly from GB or Big5 into Unicode. However, whereas there is some overlap between GB and Big5, there are also many simplified characters in GB that are not in Big5, and many traditional characters in Big5 that are not in GB. Conversion between GB and Big5 is therefore quite significant, since many simplified characters map to multiple Big5 traditional equivalents. It is easier to convert from traditional to simplified characters because there is much less ambiguity.