Next to client-side web parts, extensions are new types of customizations, SharePoint developers can build to extend SharePoint. In the developer preview you can build
- field customizers, which control the rendering of fields in list view, similar to JSLink
- list view command sets, which you can use to add options to list menu, similar to Ribbon and ECB extensions using custom actions
- application customizers, with which you can embed custom scripts to run on all pages, similar to script injection using custom actions
Additionally, the preview includes placeholders which you could compare to server-side delegate controls and an API to build dialog windows.