char are fixed length strings with a set length specified. If a string is less than the set length, then it is padded with extra characters so that it's length is the set length.
varchar are variable length strings with a maximum length specified. If a string is less than the maximum length, then it is stored verbatim without any extra characters.
nchar and nvarchar are similar to char and varchar, but they are used to store Unicode data (using the UNICODE UCS-2 character set).
UNICODE requires two bytes per character, and allows the storage and retrieval of script based languages, such as Thai, Chinese, Japanese, etc.
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