The Peoples Republic of China has over the last several decades simplified a large number of characters. Characters that have been in use for thousands of years were replaced by new or existing characters with less strokes. The official motivation is that this will help reduce illiteracy.

Mainland China now uses almost exclusively simplified characters, while most places outside China that use Chinese (Taiwan, Hong Kong, most Chinatowns, etc.) stick to the traditional character set. Any written or printed material from the 1950 or earlier will also be in traditional characters.

Many people study only one system, depending on where they are or what textbook they use. There are however some good reasons to learn both systems. Compared to becoming fluent in Chinese, learning the differences between simplified and traditional isn't that hard. Once you know both systems you will be able to comprehend Chinese no matter where it was printed. Learning the traditional set also gives you more insight into the system that is Chinese characters.

• Download a List of 250 common simplifications (pdf)

could add here:

• ways in which characters were simplified
• list of simplified radicals and components
• explanation of the merging of characters