To create a snapshot at anytime while working in the Develop module, look on the left side of the screen under the Navigator window. One of the standard menu headings is Snapshots. Until you create one, this menu will be empty. To save the current editing state as a snapshot, click the + icon to add a new snapshot. A popup window will ask you what you want to name the snapshot, and it will default to the date and time. Click OK, and the Snapshot is saved. It now appears in the Snapshots menu, as well.
Snapshots are a good tool for saving editing states in case you’d like to backtrack to an earlier version of a file. If you know you’d like to create different versions of that file, you’ll probably want to make Virtual Copies. These files are “virtual” because, like all edits made in Lightroom, they don’t duplicate the original high-resolution image file. Instead, all of the edits remain theoretical until the image file is rendered to a new file.
To create a virtual copy, right click (or ctrl-click) on any image in the filmstrip or in the preview window. On the popup menu that appears, click the line item for Create Virtual Copy.
Instantly you’ll see a new version of the image file appear in the filmstrip. Hover over it with the mouse to reveal that it’s image 2 of 2. You can now edit this image however you’d like, knowing that the original image file remains unedited and can be referred to at anytime.
To compare to versions of an image file by looking at one virtual copy next to another (or to the original), simply click on one image file and then the other, then click the C key on your keyboard, or go to the view menu and click Compare to see the two images side by side. This can be an invaluable way to study subtle differences between image files.