Explain the different normal forms (1NF, 2NF, 3NF, BCNF).

1NF: atomic values, no repeating groups (each cell has single value). 2NF: 1NF + no partial dependencies (non-key attributes depend on entire primary key, not part of it). 3NF: 2NF + no transitive dependencies (non-key attributes depend only on primary key, not through other non-key attributes). BCNF: every determinant is a candidate key (stronger than 3NF). Higher forms reduce redundancy but may impact query performance through more joins.