<p>What is meant by <code>nvarchar</code>?</p>

<p>What is the difference between <code>char</code>, <code>nchar</code>, <code>varchar</code>, and <code>nvarchar</code> in SQL Server?</p>

nchar and char pretty much operate in exactly the same way as each other, as do nvarchar and varchar. The only difference between them is that nchar/nvarchar store Unicode characters (essential if you require the use of extended character sets) whilst varchar does not.
Because Unicode characters require more storage, nchar/nvarchar fields take up twice as much space (so for example in earlier versions of SQL Server the maximum size of an nvarchar field is 4000).
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