Client Side Rendering in general is a technique to offload part of the HTML content rendering workload from server side to the client's browser. In SharePoint 2013 JavaScript-based CSR templates partly replaced XSLT (remember, you can still switch to server rendering in web part properties).
Client Side Rendering in general is a technique to offload part of the HTML content rendering workload from server side to the client's browser.
Client-side rendering is a new concept in SharePoint 2013. It’s provides you with a mechanism that allow you to use your own output render for a set of controls that are hosted in a SharePoint page (list views, display, add and Edit forms). This mechanism enables you to use well-known technologies, such as HTML and JavaScript, to define the rendering logic of custom and predefined field types.
In SharePoint 2013 JavaScript-based CSR templates partly replaced XSLT (remember, you can still switch to server rendering in web part properties), being used by default for displaying data in standard lists (exceptions: Survey and Calendar lists) - including list forms, list views and lists in Quick Edit mode, - and search results.
Developers can override some of the CSR templates to customize the visualization of data.
You can read more about customizing field using client side rendering, here. Also, You can find CSR code samples, here.
