What are the principal differences between these two. I bought a used MAC that already had Bridge but everybody seems to use Lightroom. I'm new to all of this and will be working with photos exclusively.
Bridge is a companion program used with Photoshop and other Adobe applications. In relationship to Photoshop, Bridge allows you to browse through your images and choose which of those images will be sent to Photoshop to be modified. Bridge can be configured so that double clicking on images will automatically open them in camera raw. Bridge enables you to organize your images.
Very loosely speaking, I think it would be fair to say that Lightroom is, in a sense, a combination of Bridge and camera raw. Lightroom has many tools for modifying your images. One thing that new users don't seem to understand is that adjustments made using Lightroom are not applied to the images. Instead, the adjustments are stored in the catalog (which is a database) and are applied to an image when it is displayed. This is a very simplistic comparison of Lightroom and Bridge. There are many other capabilities in Lightroom that are not part of Bridge. Lightroom is more of a complete finishing product whereas Bridge is primarily a tool for organizing and managing your images. By itself, Bridge cannot make any modifications to your images. You can not just use Bridge to do any postprocessing on your images. The way I use Bridge is I use it to choose the images that I want to open in camera raw or Photoshop or both. And I use Bridge to browse through my images after I have done the postprocessing.