First things first: PMBOK stands for "Project Management Body Of Knowledge." The definition of what PMBOK means, directly from the 1st (1996) Edition of the PMBOK Guide, is as follows: "all those topics, subject areas and intellectual process which are involved in the application of sound management principles to... projects." The PMBOK is an abstract idea meant to encompass all the knowledge project managers around the world use to successfully manage projects.
The Project Management Institute (PMI) has published five editions of A Guide to the Project Management Body of Knowledge, more commonly known as the PMBOK Guide. The most recent version came out in 2013.
Some people mistakenly refer to the PMBOK Guide as the embodiment of the general PMBOK. This guide does not contain everything in the PMBOK — you can never hope to capture everything worth knowing in one document — but it's a great place to start. It covers the context in which projects operate along with detailed processes for running projects, and it has been collaboratively compiled by many studied project managers.
The PMBOK Guide is a lengthy resource, coming in at just under 600 pages. As such, there are many, many guides to the guide. We went through what Google had to offer and sorted out some good starting places if you're just getting your hands on A Guide to the Project Management Body of Knowledge for the first time.
These resources will help you understand PMBOK, the PMBOK Guide, and how it differs from its biggest rival: PRINCE2.
17 PMBOK Learning Resources
PMBOK, THE ABSTRACT IDEA
PMI's Learning Center includes many PMBOK resources.
OVERVIEWS OF THE ENTIRE PMBOK GUIDE
Here's what's new in the 5th edition, according to PMI: • A 10th Knowledge Area has been added; Project Stakeholder Management expands upon the importance of appropriately engaging project stakeholders in key decisions and activities. • Four new planning processes have been added: Plan Scope Management, Plan Schedule Management, Plan Cost Management and Plan Stakeholder Management. These were created to reinforce the concept that each subsidiary plan is integrated with the overall project management plan.
Haven't purchased the PMBOK Guide yet? You can buy it on PMI's website.
If you're looking for a general overview of the PMBOK Guide, check out the Wikipedia page: http://en.wikipedia.org/wiki/A_Guide_to_the_Project_Management_Body_of_Knowledge
Don't trust Wikipedia? Here's another overview of the guide (which happens to be very similar to the Wikipedia page): http://www.projectsmart.co.uk/pmbok.php
Another overview of the PMBOK Guide with a slightly different breakdown: http://edward-designer.com/web/introduction-to-pmbok-guide-knowledge-areas-processes-process-groups/
PMBOK GUIDE 5TH EDITION VIDEO
More of an audio learner? Check out this overview video from IIL describing the differences found in the 5th edition of the PMBOK Guide: https://www.youtube.com/watch?v=lYaTMBMqWxU
5 PROCESS GROUPS IN THE PMBOK GUIDE
Good overview of the 5 Process Groups (skip the top section and go straight to the bottom of this page): http://www.itinfo.am/eng/project-management-body-of-knowledge-pmbok-guide/
Free management ebook covering each of the PMBOK Guide's 5 Process Groups (download link above the social sharing buttons): http://www.free-management-ebooks.com/faqpm/principles-16.htm
A presentation on the PMBOK approach based on the 5 Process Groups: http://www.itbusinessedge.com/slideshows/show.aspx?c=78357
10 KNOWLEDGE AREAS IN THE PMBOK GUIDE
Browse the content within the 10 Knowledge Areas of the PMBOK Guide: http://standardmethod.net/browse.html#/process
Free management ebooks covering each of the PMBOK Guide's 10 Knowledge Areas (click the ebooks images at the bottom to go to the downloads page): http://www.free-management-ebooks.com/faqpm/principles-17.htm
MORE TOPICS FROM THE PMBOK GUIDE
A table showing you how the Process Groups and Knowledge Areas of the PMBOK Guide work together. (Skip the top table and look at the second table.) Process Groups are along the top, Knowledge Areas are down the left: http://www.tensteppm.com/open/A6.1CompareTStoPMBOK.html
The PMBOK Guide's take on Work Breakdown Structure: https://www.workbreakdownstructure.com/work-breakdown-structure-according-to-pmbok.php
PMBOK GUIDE vs. PRINCE2 PRINCE2 is another widely-followed approach to project management, and thus, considered to be the biggest competitor to the PMBOK Guide.
A side-by-side comparison of the PMBOK Guide vs. PRINCE2 : http://www.ppi-int.com/prince2/prince2-pmbok-relationship.php
A paper comparing the PMBOK Guide and PRINCE2. It comes to the conclusion that the two are not comparable as they serve different purposes: http://www.maxwideman.com/papers/comparing/comparing.pdf
Article about how PRINCE2 can be complimentary to the PMBOK Guide: http://www.corpedgroup.com/resources/pm/HowPrince2CanComplement.asp
After going through this list of resources, you should feel comfortable with PMBOK and how the PMBOK Guide is organized. If you've done additional research on the topic, please add other articles or books you found helpful to the comments below!