In 2009, the American Psychological Association (APA) published the 6th edition of the APA Publication Manual. This revised edition contains several important differences regarding citations and reference lists, including the following:

• Retrieval Date: APA 5th ed. requires a date of retrieval before a URL in a reference. APA 6th ed. no longer requires a retrieval date unless the source is likely to change often. APA 6th ed. emphasizes the use of DOIs rather than URLs as the best way to identify an online source.
• Cities: APA 5th ed. does not require a state or country in the reference if the city is famous for publishing. According to APA 6th ed., if the city is outside of the United States, a country is always required in the reference; if the city is in the United States, then a two-letter state code is always required.
• Many authors: APA 5th ed. inserts “et al.” after the sixth author name when there seven or more authors are listed; APA 6th ed. inserts a modified ellipsis “. . .” between the sixth author and the last author, removing all names in between.
• Database names: APA 5th ed. includes database names when citing journals, if available. APA 6th ed. does not include database names; the homepage URL of the journal is used instead. If a DOI is available, it is used instead of the URL.
• DOI: APA 6th ed. puts more emphasis on the inclusion of DOIs in references as the best way to identify an online source.                           

More information can be found here.