Direct answer
Colectivos are shared 15-passenger vans that run constantly between Playa and Tulum (and stops in between like Akumal, Puerto Aventuras). 40–50 pesos one-way (~$2–3 USD). 50–60 min ride. Flag one down on the highway or board at Av. 2 Norte × Calle 2 in Playa. Cash only — pay the driver.
Colectivos are the local-and-traveler-loved shared vans that run all day between Playa, Akumal, Puerto Aventuras, and Tulum. 40–50 pesos one-way per person ($2–3 USD), ~50–60 min ride. They leave when full (every 5–15 min from terminals). Board at the Playa colectivo terminal (Calle 2 Norte × Av. 2) or flag them anywhere on Highway 307. Cash only — pay the driver on entry or exit.
Local context
Colectivos are the Mexican equivalent of marshrutkas or shared minibuses you'll find across Latin America and Asia. They exist because formal bus service is sparse on the coast — ADO runs the long-distance routes but doesn't make local stops. Colectivos fill the gap with hyperlocal frequent service. The system is privately operated, regulated by Quintana Roo state. Most colectivo drivers are men in their 30s–60s who've been driving the same route for years; they know every cenote, restaurant, and hotel stop and will drop you anywhere on the highway you ask. The system is one of the most travel-friendly aspects of the Riviera Maya — cheap, frequent, and stress-free once you've done it once.