What is the difference between UNIQUE and PRIMARY KEY constraints?

Both ensure uniqueness, but PRIMARY KEY also enforces NOT NULL and only one is allowed per table - it defines the main identifier for rows. UNIQUE allows NULL values (one null per column in most databases) and multiple UNIQUE constraints can exist on a table. PRIMARY KEY typically creates a clustered index (physical row ordering), while UNIQUE creates a non-clustered index. Use PRIMARY KEY for main identifier, UNIQUE for alternate keys.