Photoshop allows for complete manipulation of photos including compositing multiple images together. In other words it is possible to create an image in photoshop in which some or even all of the content were not actually really in a source shot.
Lightroom is a digital asset management and photo adjustment tool. It's intended for when you want the photo with the content as it was shot... but you want to adjust it for things like white balance, color, levels, highlights, shadows, etc. but you aren't actually injecting content into an image that was not actually present when you took the shot.