QUOTED_IDENTIFIER

SET QUOTED_IDENTIFIER { ON | OFF }

QUOTED_IDENTIFIER is a definition that deal with quotation mark delimiting identifiers.

• When SET QUOTED_IDENTIFIER is ON (default), identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. 
• When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all Transact-SQL rules for identifiers. Literals can be delimited by either single or double quotation marks.

Acording to the T-SQL ruels, literals must be delimited by single quotation marks:

Select "something"  -- error

Select 'something'  -- ok

, like when QUOTED_IDENTIFIER is ON.

But, in order to follow the ISO / SQL-92 rules regarding quotation mark delimiting identifiers and literal strings, we can set QUOTED_IDENTIFIER to OFF, and then:

Select "something"  -- ok

Select 'something'  -- ok

In the most of the cases, we will work according to the T-SQL ruels (QUOTED_IDENTIFIER ON).

We will set it to OFF when we will work agginst other systems, something that QUOTED_IDENTIFIER ON does not support or when we will replace existing systems to SQL Server, etc.

http://msdn.microsoft.com/en-us/library/ms174393.aspx