When Lightroom is not open, you can always see all your Lightroom images by opening to a web browser and signing in at lightroom.adobe.com. From there you can download any image as a JPEG and bring it into any application.
Some applications can see your Lightroom photos directly. For example, if you are in Photoshop, on its Home screen you can click the Lightroom Photos panel and it will list them inside Photoshop. You can then import any of them without leaving Photoshop and without opening Lightroom itself.