Photoshop is general image manipulation, layers brushes, all the power.

Lightoom is more for photography. Ingest workflow, applying small changes like colour adjustments and applying them on batches. Non-destructive edits, working with raw images without the need to "render" them into bitmaps.

After Lightoom 6 they moved to Creative Cloud, to a fairly expensive subscription. Furthermore, they did a revamp to become more inline with their mobile offerings, and started feeling even more sanded down.

Lightroom is optimized in a workflow sense for photographers, and is tightly integrated with its own digital asset management system (the photo “library”).