None! It's common – though technically incorrect – to use all of these terms interchangeably to describe the current version of the ISO 9001 standard.
Prior to December 2000 there were ISO 9001, ISO 9002 and ISO 9003 standards; all were generally referred to as ISO 9000. Eventually these standards were merged into ISO 9001. As for ISO 9001:2015, the appended '2015' indicates the year of revision. The most current revision was published in 2015.