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.
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.
Use varchar when your strings do not have a fixed length (e.g. names, cities, etc.)
Use char when your strings are always going to be the same length (e.g. phone numbers, zip codes, etc).
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