The difference and use method of varchar and nvarchar in SQL _mssql

Ask:
What is the difference between varchar and nvarchar in SQL Server, varchar seems to be an English and a Chinese character is standing two bytes, while nvarchar is an English account of a byte, Chinese characters accounted for two bytes. But what effect does this have on the ASP program?
For:
VARCHAR (n)
A variable length of n bytes and non-Unicode character data. n must be a numeric value between 1 and 8,000. The actual length of the byte that the storage size is the input data, not the N bytes.

nvarchar (n)
Variable-length Unicode character data containing n characters. The value of n must be between 1 and 4,000. The storage size of bytes is twice times the number of characters entered.

Two fields have field values: coffee and I
The varchar field occupies 2x2+6=10 bytes of storage, while the nvarchar field occupies 8x2=16 bytes of storage space.

If the field value is only English can choose varchar, while the field value exists more double-byte (Chinese, Korean, etc.) characters with nvarchar