In SharePoint 2016, there will be No More SharePoint Designer, means no new version of SharePoint designer is coming to for new version of SharePoint but we can still use SharePoint Designer 2013 with SharePoint Server 2016.
Microsoft SharePoint Designer (SPD), formerly known as Microsoft Office SharePoint Designer, is a specialized HTML editor and web design freeware for creating or modifying Microsoft SharePoint sites, workflows and web pages. It is a part of Microsoft SharePoint family of products. It was formerly a part of Microsoft Office 2007 family, but has never been included in any of the Microsoft Office suites. (Ref: Wikipedia)
SharePoint Designer is free tool which have multiple version and use to do the customizations, workflow development and branding of SharePoint Applications. SharePoint Designer 2007, SharePoint Designer 2010 and SharePoint Designer 2013 work with MOSS 2007, SharePoint 2010 and SharePoint 2013 respectively. SharePoint Designer 2013 also used for SharePoint Online to do customizations, branding, workflow development and many other tasks.
Things are still unclear what will be substitute of SharePoint Designer in SharePoint 2016, OR Microsoft going to introduce a new tool for as a substitute of SharePoint Designer OR all the customization will be performed using web browser as Microsoft already introduced Design Manager in SharePoint Server 2013 for branding so it is possibility that all customizations will be performed using browser interface.
This can be a good option as a new Browser Edge is also coming new windows version Windows 10, but anyways SharePoint Designer lovers including me will miss this product who use it for doing most of the customizations and haters who always raise issues on SharePoint Designer will also gonna miss this product.
SharePoint Foundation 2013 is the essential solution for organizations that need a secure, manageable, web-based collaboration platform. SharePoint helps teams stay connected and productive by providing easy access to the people, documents, and information that they need to make well-informed decisions and get work done. Use SharePoint Foundation to coordinate schedules, organize documents, and participate in discussions through team workspaces, blogs, wikis, and document libraries on the platform that is the underlying infrastructure for SharePoint Server. (Ref: Microsoft Download Center)