What is Client Side Rendering (CSR) in SharePoint? CSR is an API whereby Microsoft has pushed much of the process of rendering raw data as HTML off the server and onto the client (i.e. the browser) in the form of display templates (JavaScript) in SharePoint 2013 and later. It is a replacement for much of the XSLT that you may have written for previous versions of SharePoint to modify forms, views, and search results. This XSLT was processed on the server, and if you went nuts with this it could have a serious negative impact on the performance of your farm.
So, what can you do with CSR? You can develop display templates to modify list forms, views, and search results. As you might imagine, that’s a pretty big topic, so this post (and several follow up posts) will talk about overriding the rendering of fields in list forms. Eventually, I plan to post about view and search display templates as well.