Lightroom is a one-stop shop for photographic workflow management. It has
- digital asset managment
- raw image processing
- image proofing and editing
- publishing tools
all wrapped into one tidy package. And, all the tools are contextually tailored to the workflow tasks that photographers all perform.
Photoshop CS* is a one-stop graphics shop. It has tools to do pretty much anything and everything one can dream up using computer graphics. Many of these tools can be directly applied to photography, others can be adapted, and a whole bunch of others you probably would not ever utilize for strictly photographic art work.
Many of us use both tools in concert with each other. I do all my raw processing and image proofing in Lightroom, and then switch over to Photoshop to handle more advanced processing work, then back to Lightroom to publish the finished product.