Column Aliases

A synonym is used to reference a table or view by another name. An column alias is an alternative to a synonym, designed for a distributed environment to avoid having to use the location qualifier of a table or view. The alias is not dropped when the table is dropped
A column alias give a column a different heading
By default, alias headings appear in uppercase. If thealias contains spaces, special characters (such as # or $, or is case sensitive then enclose the alias in double quotation marks ("---")

SELECT FirstName AS ColumnAlias 
SELECT FirstName AS [Column Alias] 

Table Aliases