CHAR, NCHAR, VARCHAR and NVARCHAR Data Types

Table of Differences

	char	nchar	varchar	nvarchar
Character Data Type	ASCII	Unicode	ASCII	Unicode
Maximum Length	up to 8,000 characters	up to 4,000 characters	up to 8,000 characters	up to 4,000 characters
Character Size	takes up 1 byte per character	takes up 2 bytes per Unicode/Non-Unicode character	takes up 1 byte per character	takes up 2 bytes per Unicode/Non-Unicode character
Storage Size	n bytes	2 times n bytes	Actual Length (in bytes)	2 times Actual Length (in bytes)
Usage	use when data length is constant or fixed length columns	use only if you need Unicode support such as the Japanese Kanji or Korean Hangul characters due to storage overhead	used when data length is variable or variable length columns and if actual data is always way less than capacity	use only if you need Unicode support such as the Japanese Kanji or Korean Hangul characters due to storage overhead

Query that uses a varchar parameter does an index seek due to column collation sets.
Query that uses a nvarchar parameter does an index scan due to column collation sets.

Advantages and Disadvantages

Data Types	Advantages	Disadvantages
char	Query performance is better since no need to move the column while updating.No need to store the length of string in last two bytes.	If not properly used, it can take more space than varchar since it is fixed length and we don’t know the length of string to be stored.It is not good for compression since it embeds space characters at the end.
varchar	Since it is variable length it takes less memory spaces.	Decreases the performance of some SQL queries.
nchar/nvarchar	Supports many client computers that are running different locales.	If not properly used it may use up a lot of extra storage space.

Sources:

https://www.mssqltips.com/sqlservertip/4322/sql-server-differences-of-char-nchar-varchar-and-nvarchar-data-types/