Same family, different positioning
Rolex and Tudor share the same parent: the Hans Wilsdorf Foundation, which Wilsdorf established in 1944 to ensure his brands' independence after his death. Wilsdorf founded Tudor in 1946 specifically to create a brand using Rolex case quality at lower prices; for decades Tudor used ETA movements while Rolex used in-house. Modern Tudor (post-2015) uses in-house movements made by Kenissi, a Tudor-controlled manufacture; the relationship to Rolex's own movement-making division is parallel rather than shared.
What's the same
Case construction quality: Tudor uses the same case-finishing standards and water-resistance testing as Rolex. Crystal and gasket sourcing: similar suppliers. Service network: Tudor watches are serviced through the same authorised network as Rolex. Build standards: many ex-Rolex watchmakers and engineers work on the Tudor side.
What's different
Movements: Rolex Cal. 32xx (Submariner, Datejust, GMT-Master II) is parallel to but not shared with Tudor's Kenissi MT5602 / MT5612 / MT5652 family. Both are in-house chronometer-grade; the architectures are different. Bracelets: Rolex Oyster bracelet is more polished and refined; Tudor uses simpler riveted-style bracelets on Black Bay and similar references. Dial design: Tudor is allowed more visual variety (chocolate dials, snowflake hands, blue pelagos) than Rolex's tightly-controlled aesthetic vocabulary.
Which to buy
Buy Rolex for resale value, brand recognition, AD-relationship potential, and the strongest secondary market. Buy Tudor for similar build quality at 40-60% of Rolex pricing, no allocation game (Tudor is buyable at retail), and watches that wear differently (the Black Bay 58 at 39mm is smaller than any modern Rolex sports reference). The Black Bay 58 at CHF 4,000 is the canonical Tudor first watch; the Submariner at CHF 9,400 is the Rolex equivalent that costs 2.4x more.