What is index seek vs index scan vs key lookup in SQL Server?

Seek uses targeted navigation in an index, scan reads many/all rows, and key lookup fetches missing columns from clustered index.

Index Seek navigates B-tree structures directly to matching rows using predicates, making it highly selective. Index Scan reads the entire index when predicates are non-sargable or missing indexes. Key Lookup occurs in non-clustered indexes when requested columns are not present in the index leaf node, causing additional I/O against the clustered index. The optimizer chooses based on selectivity, statistics, and cost estimates.