SharePoint supports two main development / deployment techniques for creating SharePoint projects: Server Object Model and Client-Side Object Model. If you plan on doing development for the hosted version of SharePoint, i.e. Office 365 / SharePoint Online, then you CSOM is the only option. A new SharePoint CSOM has been been released for SharePoint Online / Office 365. It also allows development for Project Online (hosted Project Server). It can be downloaded using “Manage NuGet Packages” option in Visual Studio. Just search for the id “Microsoft.SharePoitOnline.CSOM” and VS will download the required files.
