PMBOK is a guide to the project Management Body of Knowledge (PMBOK Guide official book provides the practice required to increase the success rate of the projects across various domains in the practice of project management. PMI ensures the continual improvement of  the PMBOK Guide by incorporating the developments and standards according to the emarging scenarios in varous disciplines and industries. The first version of PMBOK was published in 1996 and subsequently revised every four years(roughly). The 2nd edition was published in 2000, 3rd edition in 2004, a 4th edition in 2008, and the 5th edition in 2012. The current version of PMBOK is PMBOK Guide 5th Edition.PMI scheduled to release the PMBOK 6th Edition in the 3rd quarter of 2017 which includes some significant changes according to the current trends of the industry 

Whats new in PMBOK 6th Edition?

Alignment with Agile:
With Agile project management making inroads in the current project management space,PMI ensured that the PMBOK 6th edition is aligned in conjunction with the Agile project management methodologies. In PMBOK 6th edition,each knowledge area will bear a section called "Approches for Agile iterative and adaptive Envionments" which describes how the Agile practices can be integrated in the context of traditional project management. The PMBOK appendix also contains the information about the Agile and other iterative approches. Emphasis was also laid on techinal, leadership, business and strategic areas and project management business doucments in line with the PMI Talent triagle- a balance on the technical, leadership,and strategic management. PMI published the Exposure Draft and invited opinion from the public in 2016. Based on the Exposure draft of PMBOK 6 published by PMI following changes may reflect in
the new PMBOK 6th edition. Remember, the changes are suggested in accordance to the exposure draft published by PMI and the final version of PMBOK 6th edition may have some variations. 

• Number of process groups are 5 and knowledge areas are 10. No change from previous version
• PMBOK 6 will have 49 processes instead of 47 in PMBOK 5.Three new processes "Manage project knowledge" and "implement Risk Responses" (Both under Executing process group) and "Control Resources" under Monitoring and COntrolling process group are added and One existing process "Close procurements" deleted and included the content in close project or phase in integration Management Knowledge area 
• The knowledge area "Human Resource Management" is changed to "Resource Management". Resources like machinery and material are also considered along with people(Human Resources)
• In knowledge areas like communication, Risk and stakeholder management the word "Monitor" is used instead of "Control. For examle "Control risks" process is called as"Monitor Risks", "Control communications" is now called "Monitor Communications", control stakeholder Engagement is now called Monitor stakeholder Engagement.(practically project managers will be able to monitor these areas, but have very little control)
• Time Management has been renamed to schedule management

Conclusion: Though the changes seem to be many, there is no devil is the changed curriculum. PMI would give some time window (usually 3-6 months ) to take the exam based on PMBOK 5, even after the official release of PMBOK 6th edition. Aspirants who are planning to take exam on PMBOK 5th edition can plan their preparation and take the exam!