The term "official vocaloid" is often used as an antonym of "fanmade vocaloid". In the early days of the Vocaloid boom, there were fewer vocaloids than there are now, so when an additional character was needed for fan fiction and fan art. Thus to get around the lack of voicebanks, fan made mascots, variants and derivatives were invented to act as additional "Vocaloids". Some were made to represent a producer's work and their individual settings, others were made to reflect a certain aspect within the Vocaloid fandom, some were simple solutions to fill the void left when no voicebank could cover that vocal range.
Regardless of their origins, because the popularity of certain characters, Crypton began to officially adopt these as part of their Vocaloid cast to be used commercially.  Because they appeared so often in PVs and had also begun to appear in merchandise, Vocaloid fans also began to mistake some of these characters for official Vocaloids. However, it should be noted that fanmade vocaloids are not accepted by all the Vocaloid fans.
When UTAU also came into existence, many of these previous Vocaloid habits began to disappear, as these was no longer a need to invent a unique mascot from a Vocaloid as one could make one's own representative from scratch. Though a few producers still take time to invent new characters, with UTAU, the increased number of voicebanks, including an increase in interest of Japanese producers using English Vocaloids, there is less of a call for fanmades as before. However, that does not mean that fanmades are not still being created.
Within the Vocaloid wiki you can find details of the official Vocaloids and a few notable fanmades that are popular.
As Vocaloid became popular in late 2011, "official" became known to be a term applied to only the most popular Vocaloids from Crypton Future Media, and occasionally Gakupo or Gumi due to their media coverage and heavy promotions. Other Vocaloids outside of the group are often mislabeled by uninformed fans as "not official" despite having a voicebank. Usually this is due to biased opinions of pro-Crypton fans, however it is also often spoken by fans who are not aware of the situation and licensing of Vocaloid voicebanks.