You probably already know that Magento 2 is a major new version of the e-commerce solution and that the extensions for the 1.x version were distributed over the Magento Connect platform. With the growth of the platform, some issues began to appear: there were very few controls regarding the quality of the extension and anyone was able to publish an extension, which may not work. So, Magento decided to create a new Magento Connect especially for Magento 2, which will be called the Magento Marketplace.
By the time you read these lines, the existing Connect website will most probably still be available due to a number of reasons. Because all the Magento websites cannot be migrated in such a short period, clients still have some need for functionalities. Plus, many editors of extensions work exclusively on Magento extensions, so closing Connect can be dangerous for their business.