https://english.stackexchange.com/questions/17695/any-reference-on-the-usage-of-a-backtick-and-single-quotation-mark-like-this#17696
Basically, as answered by @mgkrebbs, quotations like 'this' or `this' are faulty typographic usage introduced with limited character sets, first with 19th century typewriters then with digital character encoding. The proper (Unicode) characters are ‘these’. The same happens with apostrophe and primes (for feet and inches). One should type
“Alice’s friend told me ‘My son is 6′3″ tall.’”
instead of
"Alice's friend told me 'My son is 6'3" tall.'"
But it's easier said than done.
For reference about these various "apostrophes" I've found the following documents:
Of course, all this is mainly on English. Digressing further away from your question, you have other interesting things like the Mè'phàà language of Mexico, which uses the straight quote as a casing letter (pdf), with both a lower case ꞌ and an upper case Ꞌ version, both added in Unicode 5.1.