Unlike the percent or degree symbol, the ampersand is generally not used in editing except in very informal situations. The ampersand is used in logos and headlines for its design element, but should never be used in text. You can find more information at http://en.wikipedia.org/wiki/Ampersand.
Some style guides allow ampersands as part of a company name (Tiffany & Co.). If the ampersand forms part of a registered name, it should not be replaced with “and”. Most style guides recommend using the ampersand for common expressions like research and development (R&D). There are no spaces on either side of the ampersand when it is used that way.
In both MLA and APA style, the ampersand is used when citing sources within the body of text (Smith & Jones, 2005), but in the list of references, an ampersand can be used to precede the last author’s name when there is more than two authors (Sumner, Reichl & Waugh).
In pop culture, it can be used in common expressions like “rock & roll,” especially in a list where it is part of an item’s name, not the separator (“Rock, rhythm & blues, and hip hop”).
The ampersand is also often used when addressing a couple in writing, for example: “Mr. & Mrs. Smith”. It is commonly found in books, TV and movie titles, like Law & Order (or Mr. & Mrs. Smith). When it comes to credits for screenplays, the Writers Guild of America uses the ampersand to indicate a collaboration on the script, whereas the word “and” indicates two or more writers worked on the script at different times, i.e. one writer may have been hired to rewrite another’s work.
So despite its newfound popularity in text messaging culture, the answer to how and when to use the ampersand correctly is rarely, except in very informal situations.