High-level design (HLD) is the bird's eye view.
- Which services exist and how they talk to each other.
- Which datastore each service owns.
- Where caches, queues and load balancers sit.
- Audience: architects and reviewers deciding if the shape is right.
Low-level design (LLD) zooms into one box.
- Class and interface definitions, method signatures.
- Exact database schema, indexes and key design.
- API contracts with request and response payloads.
- Audience: the engineer about to write the code.
Always agree the HLD before drilling into LLD, otherwise you may perfect a component that should not exist.