Since Salesforce's inception, their entry-level certification for those looking to develop on the platform was the Force.com Developer 401 Certification. They also had an Advanced Developer 501 Certification exam as well, which consisted of a set of multiple choice exam questions and also a programming assignment. With the Summer '15 release, Salesforce announced they would be retiring the old Developer 401 and Advanced Developer 501 Certifications and replacing them with three new certifications. The old certifications were retired on Sept. 14, 2015 and are no longer available to acquire. Those that held the old certifications can keep them, and can also take a transition exam to attain the new certifications.
The first new certification is the Platform App Builder Certification. This certification's purpose is in its name. It is for people who excel in customizing the Platform. It allows those who tend to use the declarative capabilities (clicks instead of code) of Salesforce to hone in on those skills. The Platform App Builder Certification does resemble the old Developer 401 Certification, to a point, but there are newer Platform functions that have found their way onto this new exam. The two completely new sections of the study guide are the most obvious difference. Those would be the Social and Mobile sections. They currently hold low percentages relative to the other sections, but their appearance at all separates the Platform App Builder certification from the DEV401 certification. The emphasis on reporting was also cut in half in the creation of this new certification. The next major difference is not one that is terribly obvious upon first inspection of the study guide. The original developer certification had a section called App Design, and the App Builder certification seemed to carry that on with the new name of App Development. However, the content of both of these sections is completely different. The new certification focuses much more heavily on app deployment. This involves things such as lifecycle management, the implications of unmanaged packages, and change sets.
One key point to bear in mind is that for you to now have the word 'Developer' in your certification, you have to pass the new Platform Developer I certification. What Salesforce has done is basically split the old Advanced Developer Certification into two new certifications - Platform Developer I and Platform Developer II.
For Platform Developer I, this certification is geared heavily towards those who resort to programmatic solutions for developing on the Salesforce Platform. They also released a Platform Developer II. Both the Platform Developer I and Platform Developer II certifications contain a multiple choice component. The Platform Developer II Certification also requires that you successfully complete a programming assignment as well as an essay exam.
To learn more about the study guides and the certifications, visit: certification.salesforce.com