CAST AND CONVERT SQL/MSDN

CAST ( ColumnName as datatype)

CAST (EmpID as varchar(20))
This makes the Column EmpID as varchar from what ever datatype it was.

CONVERT (datatype, @ColumnName)

CONVERT (varchar(20), @EmpID)


We can also use in different ways like
SELECT CONVERT(nvarchar(20),HelloWorld)
This shows HelloWorld by storing it as nvarchar.

In General,

Using CAST:

CAST ( expression AS data_type )

Using CONVERT:

CONVERT ( data_type [ ( length ) ] , expression [ , style ] )