Even if you only shoot raw the reality is that we all still have a lot of rendered files on our systems too. What are rendered files? These are the photos rendered in pixels as opposed to the raw capture data produced by our cameras. Lightroom can work with 3 types of rendered files—JPG, PSD, and TIF—but there are some subtle differences in how to work with these files in Lightroom that you should know. 

Read Rendered vs. Raw in Lightroom over at peachpit.com.