Perhaps I should explain what is the difference between these two photo editing software. Both are products of Adobe. When Adobe first released Lightroom, it was advertised as a tool for photographers (which I have found to be very true indeed). Even though Photoshop has long established itself as the industry standard in photo editing, Lightroom did not unseat Photoshop, but appears to complement it very well. Adobe has cleverly filled in a workflow gap that has made "light" of post processing work (pun intended).
Here is the difference. When you use Photoshop, you work with Layers. You cannot master Photoshop without mastering Layers, and without mastering the various Selection tools. Both are not easy to master. Lightroom does not require you to work with Layers or the Selection tools, which is a big relief. Indeed, for someone familiar with photo editing, it can be learned in one sitting. In Lightroom all the adjustments are made on one layer. Since there is no Selection tool in Lightroom, you cannot do cut-and-paste. For such graphics work, one can seamlessly migrate over to Photoshop and continue working on the image.