Simplified and Traditional Chinese are used to describe two different Chinese writing systems.
Simplified Chinese was introduced in the 1950's by the Chinese government. It's an attempt to simplify the complexity of the written Chinese characters.
Simplified Chinese is used in Mainland China, Malaysia and Singapore while Traditional Chinese is used in Hong Kong, Taiwan and other Chinese communities.
To get an accurate translation of a Chinese document you'll want to know whether it was written in Simplified Chinese or Traditional Chinese. Pairaphrase supports both writing systems.