Lightroom doesn't store images in the catalog. It only references where they are, and stores information about the images and any changes you make to the images. When you edit an image in the Develop module for example, Lightroom doesn't change the original image at all; it only makes a list in its file of the changes that you made. When you Export an image from the catalog, Lightroom references the original image, adds the changes that you've made plus any export settings you've set and creates a new image file. Because Lightroom doesn't touch the original image, there's never a need to 'Save' your work in Lightroom the way you would in Photoshop for example.
Mike.